THE THERMOCOUPLE AND THE PHOTOCELL

There are two means of producing small electric currents for special purposes. One of these is the thermocouple or a thermopile. The other is the photocell, sometimes called the electric eye.

The "iron-copper" thermocouple represents an iron wire " and a copper wire, both being carefully cleaned at the end and making close contact with each other. At the point of contact between unlike metals, a current tends to flow from one metal to the other because the outer electrons in the atoms of one metal have more potential power than those in the other metal. The measure of this potential power difference is called potential difference. This potential difference depends both upon the nature of the metals and upon the temperature at the point of contact.

A number of thermocouples are sometimes connected in series. Such a combination called a thermopile is more sensitive than a single thermocouple.

The photocell generates a small electric current in response to the action of light. In one type, the light ejects electrons from a photosensitive surface upon which it falls. A photo­sensitive electrode usually consists of a thin layer of cesium or a cesium compound on a surface of silver. This is the photo­cell cathode. The anode is a metal rod or a loop, which, when the cell is in use, is connected to the positive terminal of a battery. It is the collector of electrons. The anode and the cathode are connected to short, light metal rods which extend through the base of the tube to form the support.

Electrons moving from the cathode to the anode constitute a small electric current whose magnitude is directly propor­tional to the amount of light falling upon the cathode.

Photocells perform a great number of very important serv­ices. Perhaps, the best-known use is in connection with motion pictures, where they are used in the reproduction of sound. They are also employed in television where they function in the transmission of the signal.

"Electric eyes" are also-used in factories to give automatic control of illumination, by turning lamps on or off as required. Traffic signals, the devices for testing, and recording the daily output of factories and many other types of safety devices are operated by photocurrents.

THE GALVANOMETER

The most important measuring instrument is the galvano­meter. It is used to detect and measure small electric currents. For the sake of simplicity it may bethought of as a d. с motor which can rotate only part of a turn because it has no commu­tator. It has a very low resistance.

The current to be measured passes through a coil which is wound around a soft-iron armature turned between the poles of a permanent magnet. A pointer attached to the coil meas­ures the rotation of the coil. A. c. cannot be used because the armature would no sooner start to rotate in one direction than the reversal of the current would start it rotating in the op­posite direction. Hence it would remain stationary.

In all of the experiments in which we use an ammeter, its connection in the circuit is always in series. This is necessary

because all the current to be measured has to pass through the ammeter. If we attempted to use a galvanometer instead of an ammeter in order to measure current, the galvanometer would be probably damaged.

There are two reasons why we cannot use the galvanometer directly in series. First, it is a sensitive instrument and is so constructed that a very low current is sufficient to move the pointer to the end of the scale. Let us assume that 0.01 ampere can move the galvanometer pointer the full scale, that is, to the end of the dial. If the current we are measuring is more than this amount, as it usually is, it is too great for the galvanometer to withstand, and the instrument, of course, is damaged.

Second, the galvanometer has a resistance of its own. Hence when we connect a galvanometer into a circuit its resistance reduces the very current it had to measure. As a result our measurements will be incorrect.