1942: Radio And Electronics

Television.

Broadcasting.

On July 1, 1941, the Federal Communications Commission (FCC) stated that television had proven itself technically ready for commercial operation or for selling 'time on the air,' as is done in sound broadcasting. In the middle of 1942 there were but two commercial broadcasting stations in the United States, WNBT of the National Broadcasting Company in New York and WPTZ of the Philco Radio and Television Corporation in Philadelphia. In addition there were six experimental stations in New York, two at Chicago, one at Schenectady, and one at Los Angeles. In addition to the foregoing commercial and experimental stations, there were twenty-two stations in various stages of planning and construction.

The beginning of extending local broadcasts into network service has been made by linking the New York studio with a radio path from the NBC-WNBT transmitter in New York City at the top of the Empire State Building to the General Electric Relay station on top of Helderberg Mountain, twelve miles from Schenectady. From this station the signal obtains its only boost to the main WRGB transmitter (three miles away) for rebroadcast to the Albany-Schenectady-Troy area.

There are two network links to the south of New York, one a coaxial cable to Philadelphia, owned by the American Telephone and Telegraph Company, through which the Republican National Convention in 1940 was sent to New York for television rebroadcast. Also there is a relay station W3XP at Wyndmon, Pennsylvania, 82 miles from New York and 8 miles from WPTZ in North Philadelphia, from which Philco receives the NBC programs in New York for rebroadcasting. With but little effort and material the network can readily be extended to Washington where it can be linked with a television broadcasting station in that city.

Early in the year these stations and networks were used to assist in developing preparedness, being used as the chief method for training the 54,000 wardens in the boroughs of New York City alone, in their different duties. Television receivers were installed in the classrooms of all the police stations, and the wardens assembled in these classrooms to receive instructions. It was felt that this type of instruction was not only more effective than any other but that it was the most economical method. The instruction could be given by the very best instructors, of which there are but a few, and thousands of wardens and citizens could be reached directly by this method. Similar training for Red Cross workers is progressing.

Considerable was learned after television had been 'commercial' for six months. It is estimated that 12.7 per cent of the country's population is within range of commercial service and an additional 6 per cent within range of experimental service. Another 3.6 per cent is within range of stations which soon will be, or are already, in operation. Hence plans and programs can be planned on their being accessible to 22.3 per cent of the nation's population. It is estimated that there are approximately 6,200 receivers in the areas covered. The reason that the number is not greater is due to the difficulty and the time required to convert existing receivers to conform to the new FCC broadcasting standards. Also the war conditions have prevented any organized promotional work on the part of the manufacturers. An analysis shows that in the New York area the percentages of viewing tubes according to diameter are as follows: 12 inches and more, 49.3; 9, 18.8; 5 and less, 31.9. The prices of receivers ranged from $100 for kits with 3-inch screens to approximately $400 for large receivers with 12-inch tubes.

Since July 1, 1941, WNBT has been broadcasting a minimum of fifteen hours a week, this minimum being required by the government for commercial operation. Three basic sources for programs were used, a 3-camera live-talent studio, a film-transmitting studio, and portable or mobile equipment for pickups. The hours use of the mobile equipment declines in winter and in poor weather. Such scenes as boxing, Columbia University football games, tennis, hockey, swimming meets and ship launching were picked up, televised and transmitted. There is no doubt that concentration on the war effort is greatly impeding the television programs and the growth that it would have under normal conditions.

The foregoing experiences indicate, however, that when the war situation has passed there will be popular growth of interest by the public in television.

Heat-Treated Television.

Television programs originating in the new General Electric Company studio in Schenectady will be relayed to the main transmitting station in the Helderberg Mountains, 12 miles south of Schenectady by a novel antenna encased in an electrically heated wooden box, 13 ft. high and 4 ft. square. The box and antenna are mounted on top of a 128-foot tower. The object of enclosing the picture-wave antenna in a heated box is to protect it against rain, snow and sleet which might affect the transmission. Heating is necessary to prevent moisture freezing on the antenna, and wooden construction rather than metal is used to avert field distortion.

To beam the pictures as directly as possible to the station in the Helderbergs a four-unit system stacked vertically was built. The vertical stacking assists in controlling the wave direction and the degree of beaming improves with an increase in the number of units. Each unit consists of two parallel dipole antennae connected at their feed point by a quarter-wave coupling.

The F-M (frequency-modulated) sound waves accompanying the picture waves will be relayed to the transmitting station in the Helderbergs by means of a dipole antenna on top of the wooden box.

Frequency Modulation.

Frequency Modulation for Telegraph Lines.

The Western Union has announced the first application of frequency modulation (FM) to wire telegraph circuits. This change is made in order to eliminate or minimize disturbances caused usually by meteorological conditions which have pronounced adverse effects on the usual amplitude-modulated carrier currents. As is well known, it is the custom of telephone and telegraph companies to send modulated carrier-current waves of several ranges of frequency over a given wire channel. At the receiving end, each of a number of receivers is tuned to a particular carrier frequency, and responds only to the one to which it is tuned. Each carrier wave is modulated by the low-frequency telegraph signal. This system is similar to radio broadcasting; each sending station sends out its modulated carrier wave adjusted to an assigned frequency, and radio receiving sets are tuned to respond to the frequency of the station to which it is desired to listen. With amplitude modulation (AM) the intensity of the carrier wave responds to the low frequency of the telephone or telegraph message; with frequency modulation (FM) the frequency or the pitch of the carrier is varied in accordance with the audio frequency modulation of the telephone or telegraph message. It is found that frequency modulation is affected little if any by such disturbances as static, so that the telegraphic transmission will be much less subject to external troubles than occur with amplitude modulation.

Frequency modulation has been installed on four carrier-current systems connecting New York City with Buffalo, Chicago, Washington, and Ottawa. It is planned to use FM with future carrier-current systems.

Frequency-Modulated Control for Sound Films.

The field of frequency-modulation has now been extended to increase the volume range of sound pictures. In spite of recent improvements in sound-picture recording, the available volume range still falls far short of meeting the volume range requirements of modern sound pictures. Volume expansion at the desired points has been accomplished by means of a 5-mil (0.005-inch) frequency-modulated control track the operation of which is described in a Journal of the Society of Motion-Picture Engineers, by J. G. Frayne and F. P. Herrnfeld. The usual recorded sound is fed into a single channel, amplified and applied by a light valve to form a 0.076-inch sound track in the usual manner. Simultaneously, the control frequency, generated by a variable-frequency oscillator, is applied to the 5-mil track by a light valve, the desired volume, which is heard over the monitoring system, being controlled by varying the control frequency or the frequency of the oscillator. In reproduction similar frequency variations derived from the frequency-modulated track control the desired volume range of the reproduced sound. The 5-mil track is located on the film between the sound track and the picture area.

Tubes.

Radio Transmitting Tube.

The General Electric Company has developed a compact radio transmitting tube, designated as GL 8010-R, especially suitable as an ultra high-frequency power amplifier. The tube is only approximately 3½ inches long, has a small diameter, and yet has a maximum plate input of 100 watts. Both the anode and cathode are provided with circular discs or coolers for forced-air cooling. The coated cathode is heated by electron bombardment from an auxiliary filament. The parallel-plate electrodes are closely spaced to facilitate neutralization. Grid-plate capacitance is 1.5 micromicrofarads; grid-cathode capacitance is 2.3 micromicrofarads; and plate-cathode capacitance is 0.07 micromicrofarads. Low-lead inductance is provided by the disk-type terminals.

When used as a Class C radio-frequency amplifier, the tube has a maximum d-c plate voltage of 1350 volts; maximum plate current is 150 milliamp; and maximum plate dissipation is 50 watts. The tube has an amplification factor of 30.

Iron-Glass Seal.

Tight scale between iron and glass for wires leading into certain types of vacuum tubes is now being made, thus eliminating the need for nickel and cobalt which are critical war materials. This is the result of a new development of Dr. Albert W. Hull and Dr. Louis Navias of the General Electric Research Laboratory, who have just been granted a patent for their invention.

Owing to the need for cobalt and nickel, previously used, and their limited supply, Dr. Hull and Dr. Navias devised a series of glass compositions which can be used with iron and certain iron alloys. One consists of 45 per cent silicon dioxide, 14 per cent potassium oxide, 6 per cent sodium oxide, 30 per cent lead oxide, and 5 per cent calcium fluoride. The rate of expansion of these glasses is very nearly equal to that of iron.

When glass containing lead is sealed in contact with iron, some of the lead irons migrate to the iron, which weakens the joint and may produce a leak. This difficulty is overcome by another patented invention. A thin layer of lead-free gas is placed directly over the iron wire and the lead containing glass is sealed to this. The thin glass layer prevents the lead reaching the iron, yet is not thick enough to crack and permit air to enter.

Miniature Vacuum Tubes.

Two new miniature tubes, 361A and 362A, not larger than the little finger and half as long, were developed primarily for the new Western Electric Orthotronic audiphone. A unique feature is their extremely fine filaments which permit operation on a single dry cell. The tubes are pentodes. Five leads pass through the press. In the audiphone they are connected by binding posts to avoid the increased length which a socket would require. The 361A is used in a resistance-coupled amplifier with a gain of 37 decibels, and the 362A is a power tube with a normal output of 9 milliwatts. Both tubes operate without bias. Although introduced primarily for audiphones, they are suitable for other applications, particularly where space is at a premium.

Rotating-Anode X-Ray Tube.

In the use of X-rays for physiological studies it is desirable to reduce the exposure to as short a time as is possible, since the involuntary movement of the organs blurs the photograph. For example, it is desirable that the exposure for lung techniques be about 1/30 sec., for gastro-intestinal radiography, 1/5 sec. In order that the time of exposure may be reduced, it is necessary to increase the milliamperes (Ma.) through the tube. There has been a gradual increase from 10 Ma. to 30 to 100 to 350 and now the current is 500. However, the current represents the electron carriers of electricity which bombard the cathode, resulting in X-rays being emitted by the bombarded area. As the current has increased it has become necessary to increase on the anode the focal area of bombardment from 1.5 mm. square to 5.2 mm. square at 500 Ma. The larger focal areas mitigate against sharpness of the image on the photographic film and thus tend to offset the reduction in blurring due to the shorter time of exposure. The need for a larger area with the larger currents is due to the fact that too great an energy concentration on a small area of the anode injures the anode. A group of engineers of the General Electric Company have overcome this difficulty by using a rotating anode. The anode consists of a solid tungsten disc cast into a massive cylinder of black-coated copper. There is a bevel on the face of the disc and the filament stream is offset from the center of the tube so that the electron stream strikes this bevel and the X-rays leave the tube sidewise. (The disc has the appearance of an automobile poppet valve.) The rotor of an induction motor is integral with the copper cylinder and outside the tube surrounding the rotor there is a split-phase stator of a single-phase induction capacitor motor. The rotor is driven by induction at 3400 r.p.m. so that the electron stream is no longer concentrated on a single stationary area, but over a rotating strip represented by the circular distance around the bevel. For example, due to the angle of the bevel (15 degrees) the bombardment occurs over an area 7.5 mm. wide and 150 mm. long, yet the sidewise projected area is only 2 mm. square. Hence, a large energy concentration is combined with a small effective focal area, giving a sharp image and permitting a very short time of exposure.

1,000,000-Volt X-Ray Units.

A complete portable unit, three feet in diameter, four feet high and weighing 1,500 pounds, containing a 1,000,000-volt X-ray unit, has recently been developed by Dr. Ernest C. Charlton, head of the X-ray division of the General Electric Research Laboratory, together with his associate, W. F. Westendorp. Previous 1,000,000-volt units were as large as a house and of course were fixed in position. This was no great disadvantage in hospitals where the patients could readily be brought to the apparatus. However, in industrial plants it is necessary that the apparatus be portable so that it may be brought to the vicinity of the masses of steel weighing tons, which are to be examined for flaws, blowholes, etc. The units of the new apparatus are portable.

Two factors enabled this remarkable development. A resonant transformer developed by Mr. Westendorp eliminated the heavy iron core usually employed. Forty pounds of Freon gas as an insulating material was substituted for the usual oil weighing 5 tons. Furthermore, the transformer is placed within the unit, thus eliminating external high-tension wires. The resonant transformer steps up the voltage to 1,000,000 volts and this voltage is applied to the tube located at the center of the coil where the iron core ordinarily would be, the voltage being applied in twelve stages of 85,000 volts each. The secondary winding consists of 120 separate sections wound on insulating discs. These insulating discs are piled one on top of another and the contact between the windings of the individual discs is made by special springs. The 1,000,000 X-rays can penetrate 8 in, of steel and photographs are taken in a few minutes, which otherwise would require hours or be impossible. These units will speed war production. Forty are already in successful use.

Scanning Electron Microscope.

At the Cleveland convention of the Institute of Radio Engineers, held in June, V. K. Zworykin, the inventor of the electron microscope, together with J. H. Hillier and R. Snyder, described a new development in the electron microscope. It consists of an electron microscope of the scanning type designed to examine the surfaces of all material, including opaque objects and utilizing the high resolving power of the electron beam. The usual type of electron microscope has been limited to 'transparent' subjects by permitting the beam to pass through them. In the scanning microscope the specimen is moved mechanically in such a way that each point of its surface is scanned in a systematic manner by an electron probe in much the same manner as is done with television scanning. The secondary electrons which are emitted from the point of the specimen bombarded by the electrons of the probe are accelerated and projected on a fluorescent screen. The intensity of light emitted by the fluorescent screen varies in accordance with the secondary emission properties of successive points on the specimen. This light signal is converted into an electrical signal by means of a multiplier phototube and is then synthesized in a printed picture by an amplifier and facsimile printer system.

An experimental model of the scanning microscope has been constructed and has been successful in producing images of etched metal surfaces at magnifications as high as 10,000 diameters with a resolving power considerably better than 50 microns.

Electron-Tube Ice Indicator for Airplanes.

For a number of years air-operated pulsating rubber boots on de-icers have been used on airplanes to break up the ice on the leading tips of the wings before the accumulation can alter the shape of the wing and hence the characteristics of the air foil. The operation of such devices is rather critical. If the ice is too thin, it breaks up, but most of it still clings to the wing forming a matrix for further accumulation of ice. If the ice is too thick, the de-icing operation is not effective in removing it. To enable the pilot to determine the times that the de-icer should be operated, the Minneapolis-Honeywell Regulator Company has developed an ice-thickness indicator which employs electron tubes and electric circuits. When the thickness of the ice reaches one-eighth of an inch, the optimum thickness for removal, the pilot receives an indication and he causes the de-icer to operate, or the device may be made to operate automatically. Because of war conditions, it is not possible to describe the electric circuits and mode of operation of this apparatus.

Transmitters and Receivers.

Hand Set Radio Transmitter and Receiver.

The Weltronic Corporation of Chicago has made available a completely self-contained combination radio transmitter and receiver that is in the form of the hand-set of a French telephone and is not much larger. It weighs only four pounds. The set is designed for governmental services and agencies including municipal divisions, fire and police departments, railroads and other transportation agencies, all subject to licensing by the Federal Communications Commission. The set operates with a built-in battery having a continuous rating of 8 hours which is equivalent to from a week's to a month's operation under normal service conditions. The sets have a range of more than a mile over land. They are designed for operation on a single wave length, thus requiring no tuning-in service, although they may be adjusted over their tuning range of from 112 to 300 megacycles through an externally accessible screw adjustment. There is also provided a volume control and an adjustable short fish-pole type aerial.

When the toggle switch is thrown to the 'ON' position, the unit is receiving; to talk, it is merely necessary to pull the selector finger lever down against light spring pressure. Releasing the selector lever switches the unit back to receiving again. Hence, the sets may operate without a central transmitting control station.

Pocket-Type Receiver.

At the convention of the Institute of Radio Engineers held at Cleveland in June 1942, W. J. Brown of the Brush Development Company of Cleveland, Ohio, described a pocket radio receiver of such size that it can readily fit into the heart pocket of an ordinary suit of clothes. It is provided with a specially designed hearing-aid unit which fits into the ear. In fact this portable receiver resembles the usual hearing-aid except that the batteries are incorporated as part of the receiver rather than being a separate unit as is usual with hearing-aids.

The unusual features of the set are the use of four tubes with permeability tuning, the use of a hearing cord together with the receiver as an antenna and the very small dimensions of the receiver. The filaments are heated by a 1.5-volt dry cell and the B-battery consists of a 30-volt dry-cell unit.

Traffic Control.

Pennsylvania Turnpike Radio Traffic-Control System.

There has been installed on the Pennsylvania six-lane turnpike a $300,000, ultra-high frequency, amplitude-modulated and frequency-modulated system which provides two-way communication between toll booths at ten interchanges, six maintenance buildings, portals of seven mountain tunnels, turnpike headquarters, and police cars patrolling the 161-mile turnpike. The installation is probably the most flexible radio-traffic control system in the world and also involves the most advanced radio engineering. Space limitations permit only a brief description of the system. For complete description see Electronics, May 1942, p. 34.

The general operation of the system is as follows: patrol cars and all fixed stations use amplitude-modulated (A-M) transmitters which operate at 37.5 megacycles. These transmitters control automatically the operation of, and modulate, a chain of 116-119 megacycle (MC) unattended repeaters located on six hilltops along the 161-mile turnpike. On certain of these hilltops there are frequency-modulated transmitters operating at 33.94 (MC) megacycles. These are switched on simultaneously with the repeaters. Signals from the frequency-modulated (F-M) transmitters are picked up by patrol cars.

The heart of the system is the hilltop amplitude-modulated repeaters. When any one of these repeaters is put into operation by remote control, it automatically switches on all the repeaters both to the east and to the west. In order to minimize interaction between repeater transmitters the repeater transmitter frequencies are 'staggered' within the 116-119-megacycle band, which is the lowest and therefore the most efficient ultra-high frequency on which the Federal Communications Commission (FCC) permits unattended relaying. Interaction is also further reduced by using alternately vertical and horizontal antenna polarization. The foregoing repeater-transmitters and receivers carry amplitude-modulated signals from end to end of the turnpike, the repeaters operating in unison, when any one repeater is put into operation.

Any individual repeater station, in addition to the 116-119-MC receiver, is also equipped with a 37.5-MC amplitude-modulated (AM) receiver which switches on receivers when they are excited by signals from 37.5-MC amplitude-modulated transmitters installed in patrol cars, interchanges, etc. Any repeater station must be able to initiate operation of the entire chain when excited by any 37.5-MC modulated signal.

The repeater transmitters convey speech as well as carrier-control currents. Any fixed station such as an interchange or maintenance building may be equipped with a 116-119 MC receiver tuned to the nearest repeater. Since the repeaters all do not use the same transmitter frequency between the 116-119-MC band, reception from these frequencies would be impractical for patrol cars moving along the turnpike. It would be necessary for patrolmen moving along the turnpike to retune their receivers when leaving an area covered by a repeater of one frequency and entering an area covered by the next repeater operating at another frequency. To obviate this difficulty frequency-modulated (F-M) transmitters uniformly tuned to 33.94 MC are also installed in the buildings housing four of the mountain-top repeater transmitters. These F-M transmitters are always placed in operation simultaneously with the 116-119-MC amplitude-modulated repeater transmitters located in the same building. All car receivers are frequency-modulated units tuned to 33.94 MC, as are also more than half the interchange and maintenance building receivers.

Frequency modulation rather than amplitude modulation is used for patrol car receivers, since with amplitude modulation heterodyning between the 33.94-MC transmitters would occur when patrol cars are midway between stations. With frequency modulation, this does not occur, for a frequency-modulated carrier operating on a given frequency blankets out a weaker signal similarly modulated. The frequency-modulated field between stations is so smoothly 'blended' that patrolmen cannot tell from reception when they are passing from an area covered by one station to that covered by another.

The operation of the system is illustrated by an example of a patrolman desiring to summon an ambulance located near some definite ambulance station. He listens in to be certain that the 161-mile party line is not in operation, depresses his push to-talk button, waits 15 seconds to be certain that all the repeater station relays are energized and calls. His 37.5-MC amplitude-modulated signal excites a receiver on the nearest mountain top and electrical relays energized by the output of this receiver switch on the 116-119-MC repeater at this station in the manner already described. By remote control all other 116-119-MC repeaters in the chain are simultaneously switched on. The repeater nearest the ambulance station feeds audio frequency through a cable down to the building where the attendant receives the message and dispatches the ambulance to the spot designated by the patrolmen.

The power for the sets in the cars is derived from a high-voltage dynamotor installed in the rear trunk compartment and the power supply comes from a heavy-duty charging generator and a 200-ampere-hour storage battery. The power at the several fixed stations is stepped down in voltage from local high-voltage 60-cycle power lines. To guard against interruption of the service several of the stations are equipped with emergency storage-battery reserve. All interchange utility buildings are equipped with a one-kilowatt gasoline-driven emergency generator which automatically cuts in if the 60-cycle service fails.

All interchange ticket booths are equipped with a handset and speaker making two-way radio communication possible between any ticket collector and any other point on the entire turnpike system, including mobile patrol units. In addition all ticket booths and interchange offices are equipped with burglar alarms designed to foil holdups. By means of a concealed manual switch a ticket taker transmits a 30-second film-recording warning over the entire communication system and simultaneously mutes the speakers at the attacked booth so that conversation concerning the rescue may be conducted without the intruder being aware.

Radio Control of New York Passenger Traffic.

To expedite the dispatching of trolleys and buses, the Brooklyn and Queens Division of the New York Transit System has equipped a fleet of twenty patrol cars with two-way police radios. This new system relays emergency calls to the cruising patrol cars in less than forty seconds. Of 120 street supervisors, 20 drive the radio patrol cars, each car being operated by one man. The cars carry fuses, cable, a 10-ton jack and other emergency equipment. In addition there are receiving sets on five emergency trucks, on department automobiles and one on a light truck and one on each of the track patrol cars.

The dispatching equipment consists of a 50-watt transmitter with its antenna on top of a building 240 feet above the street level and near the heart of the patrolled area. The equipment is operated by remote control from the dispatcher's office at the system headquarters. Between 50 and 100 calls are issued during the average 24-hour day. By means of this system several tie-ups of traffic, such as are caused by fires and other accidents, have been averted by dispatching the patrol cars in time to strategic points so that the cars were rerouted.

Blind Landing of Airplanes.

William Lee Clemmer of Monroe, Wisconsin, has been granted U. S. Patent 2,269,437 in which the blind landing of airplanes is facilitated by means of a low-power, low-frequency transmitter. An aviator when at a safe height, as 6,000 feet, in the neighborhood of the landing field is enabled to spiral down until he reaches an altitude from which he can glide into the field at a safe gliding angle. This is accomplished by indications on the instrument board which indicate the angle between the direction of the transmitter and the horizontal. The accuracy is such that at a distance of half a mile from a transmitter of six watts output at 375 kilocycles, the angle was indicated to within one-sixth of a degree.

The transmitter produces an 'inductive field' in the immediate vicinity of the antenna that surrounds it equally in all directions (i.e., it is not a beam) and very little energy is radiated away in the form of waves. Since the intensity decreases rapidly with the distance, it extends practically only a few miles. The pick-up on the plane is actuated only by the inductive effect of this field, and not by radio waves. This field does not interfere with high-frequency beams or other radio aids to the aviator.

Electronic Applications.

Electronic Profilometer.

It is desirable to be able to measure surface quality or the roughness of a surface finished by abrasives, by turning, by milling, plating, or grinding. This measurement may be made by means of an electronic profilometer designed by the Physics Research Company of Ann Arbor, Michigan. It is possible to read on a calibrated instrument, surface irregularities from 0.5 to 1,000 microinches above and below an average center line or reference point. An instrument with a special scale permits readings between 0.1 and 1 microinch. The principle of operation is that a stylus consisting of a diamond mounted on the end of a duralumin rod is moved over the surface. Parallel springs restrict the motion of the rod so that it is practically perpendicular to the surface. The upper end of the rod terminates with a coil of about 40 turns which is located in the field of a permanent magnet. When the stylus is drawn over the surface to be investigated, it transmits the vertical motion due to surface irregularities to the rod, which in turn causes a movement of the coil in the permanent-magnet field. The output of the coil is proportional to the velocity of the coil and is equal to about 2 microvolts per microinch at a frequency of 60 cycles per second reference frequency.

The small electromotive force caused by the motion of the coil is amplified, rectified and the resulting current produces indications proportional to the surface irregularities on a direct-reading, direct-current instrument.

The diamond point is positioned midway between two 'skids' which form the guiding device as the stylus moves over the surface. The skids thus form the basis for the center or reference line. The instrument is calibrated by placing the stylus in contact with a brass block which vibrates vertically through a known amplitude. The relation of indications of the instrument to the known displacement gives the calibration.

Electronic Speed Cop.

On the highway near St. Paul and Minneapolis, Minnesota, is an illuminated sign which at night is switched on as the motorist approaches and a second or two later the speed of his car is flashed in lighted numbers large enough to be seen clearly by the driver. The sign also warns of a danger spot ahead. The sign is operated as follows: as the car passes a certain point on the highway it interrupts a beam of invisible rays. A photoelectric relay switches on the sign and starts an electronic timer which measures the speed of the car. As the car passes a second electric eye the timer calculates the speed and flashes the speed in lights as a warning to the driver. The electrons, travelling with the speed of light, transmit the message to the signboard in ample time to warn the fastest driver.

Detection of Metal Fragments with Radio Frequency.

It is now possible to locate and remove pieces of metal from wounds by means of a radio frequency device. In the old method such metal was located by means of probes and X-rays. The apparatus was devised by Colonel John J. Moorehead of the U. S. Army, professor of traumatic surgery at the Army Post Graduate School in New York City. The apparatus consists of a radio-frequency circuit mounted in a box about one by one by two feet weighing about ten pounds, with a movable coil or a capacitor connected by a wire and enclosed in a steel finger about one-half inch in diameter and twelve inches long, which is water tight. The connecting wire is rubber and can be detached so that the finger may be sterilized by boiling. One knob of the indicator adjusts for fragments of steel and the other for non-ferrous metals. As the indicator approaches a piece of metal a deflection on a milliammeter occurs.

In use the indicator is passed above or around the would in two planes at right angles to each other. At the points of greatest deflection, marks are made on the flesh. The intersection of the projection of these marks indicates the location of the fragments. If this is not sufficient, the indicator may be introduced directly into the wound. This apparatus reduces the time necessary to locate a fragment to a fraction of the time formerly required.