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.

FARADAY'S DISCOVERY

Although for certain purposes we still employ batteries to a limited extent to generate the electric current, the usual procedure today is by electromagnetic induction. Great genera­tors in our power stations, driven by powerful turbines, oper­ate through the relative movement of conductors and magnets on a principle discovered in 1831 by that remarkable man, Michael Faraday.

A bookbinder's apprentice in London, Faraday was a clever boy. In the early part of 1812 he was given tickets to hear a course of lectures by Humphry Davy at the Royal Institution. At the end of the course he wrote up his notes on the lectures, bound them and sent them to the lecturer with a request that he should be employed as assistant. A few months later, at the age of twenty-two Michael Faraday was appointed to a post at the Royal Institution at 25 shillings a week. Thus, he started on that remarkable career which lasted for nearly half a century, during which he laid the founda­tions for much of our present electrical age. He became a skilful experimenter and an enthusiastic lecturer.

During the ten years or so before his great discovery, many investigators had interested themselves in the connection be­tween electricity and magnetism. It had been definitely established by Oersted's experiment that magnetism could be produced from the electric current. Why, then, could not the process be reversed and the electric current produced from magnetism?

The fulfillment of Faraday's hopes came in the year 1831 as a result of his experiments in the laboratory at the Royal Institution. We can read in his "Laboratory Notes" how, day by day, he carried on different experiments with wire and coils, permanent bar magnets and magnetic needles, with varying results.

On October 17, 1831, he discovered that if he had a coil of wire connected to a galvanometer and inserted a magnet into the coil, he obtained a deflection on the galvanometer. The coil consisted of eight windings of copper wire each 27 feet long, the windings being connected in parallel. When he was inserting one end of the magnet into the coil, he noticed that the deflection of the galvanometer continued only for a short time and stopped as soon as the magnet was completely inserted. No current was generated while the magnet remained stationary. When it was taken away, there was a second gal­vanometer deflection, but this time in the reverse direction. In both cases, however, there was a current only during the time that the magnet was moving.