Showmen's Trade Review (Apr-Jun 1945)

42 SHOWMEN'S TRADE REVIEW June 23, 1945 How Thermo-Couple and Photo Cells Function as Electromotive Force Article Xo. 12 — Fundamentals Of Projection Continued By Tom McNamara lATSE 505 The Thermo Couple as a Producer of an Electromotive Force. This device consists of two pieces of dissimilar metals brought into firm contact with each other and the junction protected against losing or absorbing any heat from external sources. The junction is made in such a manner as to prevent either inductive or capacitive effects. Suppose we arrange the junction in the form of the letter "X." (See illustration.) The lower points of the "X" will be the terminals of an electrical connection. The upper points of the letter "X" will be where we connect a sensitive volt or ampere meter. Each diagonal of the "X" is a solid piece of metal firmly in contact with each other at the cross over. The current enters the lower left-hand point and emerges from the lower right-hand point. A small temperature rise occurs at the point of contact (or junction) due to the resistance of the connection. Since we are using two dissimilar metals and their specific resistances differ there will be a greater electronic activity in one metal than in the other and we will have a difference of potential between the two upper points of the "X." Our meter connected at these points will indicate this potential difference. The meter will be calibrated in amperes. The meter used is a direct current instrument and will read in direct current values regardless of the nature of the current flowing in the lower branch of the "X." Since there is no inductive or capacitive effects in the junction the meter will indicate accurate values even at very high Radio Frequencies where the usual meters lose their accuracy and sensitivity. Since an Alternating Current Volt or Ampere are said to be equal to a Direct Current Volt or Ampere when they both develop the same amount of heat when causing current to flow through equal resistances, and since this heat will develop the same amount of potential difference across the junction the "Thermo-Couple" Meters read either Direct Current or Alternating Current from poiver frequencies up to and including Radio Frequencies. Since the amount of heat that is developed in the couple will be proportional to the square of the current flozving through the junction this meter is often referred to as a current-squared meter. However the meter scale is calibrated in amperes. The usual THERMO-COUPLE FIG. 1 PHOTRONIC CELL WVWWVvW Shunt Fiq. 2 Glass Envelope Cathode Anode Cathode Support Metallic Electrode Translucent Metallic Film V Liqht Sensitive Element ^ Base Metal and Electrode Fiqupe 3 TUBE ^ . N^x-J^vAnode , ( -A-Vacuum + ^ Cathode + 1 Meq. J\.^ (load resistor) Ammeter shunts are used whenever it is desired to read currents of considerable value. The Thermo-Couple itself must be protected against temperature changes otherwise its calibration will be affected. You could easily produce enough current from the couple by subjecting it to the heat from the flame of a match or the contact of a heated soldering iron. Therefore these couples are usually sealed in a compound that does not easily liberate its temperature or absorb temperature from the surrounding air. The couple, when used, should be placed in such a position as to be free from the heat liberated from such devices as transformers, vacuum tubes and resistors. PhotoElectric Action Photo-Electric Action as a source of Electromotive Energy. There are many different types of Photo-Cells. In this session we will consider only two of them as they are used in the theatre. The first to be considered is called the Photo-Voltaic tube. In this tube an actual voltage is created between the elements of the cell whenever light is permitted to fall on one of its elements or electrodes. The PhotoVoltaic Cell depends upon an oxide or an electrolyte of certain composition in order to develop an EMF (Electromotive Force) within the Cell. This EMF is developed by the light falling on the oxide or electrolyte surface and causing an electronic disturbance that is greater on one surface than on the other. Jiist as the difference in sensation of sunlight falling on one side of our body causing us to feel warmer on that side than on the other. Certain oxides are more active in this respect than others and needless to say those that are most active are the ones selected. The Weston Photronic Cell is an excellent example. This consists of an iron electrode coated with Iron Selenide over which is spread a thin film of metallic substance that will pass light to a generous degree. A thin metallic frame is brought into contact with the metallic film and serves as one electrode of the cell and the iron base serving as the other electrode. (See Fig. 1.) To these two electrodes a sensitive meter is connected and the current developed by the cell flows through the meter and deflects the pointer. This deflection will be in proportion to the sensitivity of the Cell, the amount of the light and the sensitivity of the meter. As the light strikes the Iron-Selenide an {Continued on Page 54)