registers and the quantity of alternating current, which after all is the quantity which is really to be measured. How the rectification is
accomplished will be referred to again in the chapter on Wireless Telegraphy.
But to return to the thermo-galvanometers, as those are termed which ascertain the strength of a current by the heat which it produces, the simple little contrivance of Sir William Snow Harris has more elaborate successors, of which perhaps the most interesting are those associated with the name of Mr W. Duddell, who has made the subject largely his own. Besides their interest as wonderfully delicate measuring instruments, these have an added interest, since they introduce us to another strange phenomenon in electricity.
We have just noted the fact that electricity causes heat. Now we shall see the exact opposite, in which heat produces electrical pressure and current. And the feature of Mr Duddell's instruments is the way in which these two things are combined. By a roundabout but very effective way he rectifies the current to be measured, for he first converts some of the alternating current into heat and then converts that heat into continuous current.
If two pieces of dissimilar metals be connected together by their ends, so as to form a circuit, and one of the joints be[38] heated, an electrical pressure will be generated which will cause a current to flow round the circuit. The direction in which it will flow will depend upon the metals employed. The amount of the pressure will also depend upon the metals used, combined with the temperature of the junctions. With any given pair of metals, however, the force, and therefore the volume of current, will vary as the temperature. Really it will be the difference in temperature between the hot junction and the cold junction, but if we so arrange things that the cold junction shall always remain about the same, the current which flows will vary as the temperature of the hot one. The volume of that current will therefore be a measure of the temperature. Such an arrangement is known as a thermo-couple, and is becoming of great use in many manufacturing processes as a means of measuring temperatures.
In the Duddell Thermo-galvanometers, therefore, the alternating current is first led to a "heater" consisting of fine platinised quartz fibre or thin metal wires. Just above the heater there hangs a thermo-couple, consisting of two little bars, one of bismuth and the other of antimony. These two are connected together at their lower end, where they nearly touch the heater, but their upper ends are kept a little apart, being joined, however, by a loop formed of silver strip. This arrangement will be quite clear from the accompanying sketch, and it will be observed that the loop is so shaped that the whole thing can be easily suspended by a delicate fibre which will permit it to swing easily, like the coil in a mirror galvanometer.
It is indeed a swinging coil of a galvanometer formed with a single turn instead of the many turns usual in the ordinary instruments,
and it will be noticed from the sketch that there is a mirror fixed just above the top of the loop.
This coil, then, with the thermo-couple at its lower extremity, is hung between the ends of a powerful magnet much as the fibre of the Einthoven Galvanometer is situated. The alternating current to be measured comes[39] along through the heater. The heater rises in temperature. That warms the lower end of the thermo-couple. Instantly a steady, continuous current begins to circulate round the silver strip which forms the coil, and that, acting just as the current does in the ordinary galvanometer, causes the coil to swing round more or less, which movement is indicated by the spot of light from the mirror. A current as small as twenty micro-amperes (or twenty millionths of an ampere) can be measured in this way.
Mr Duddell has also perfected a wonderful instrument called an Oscillograph, for the strange purpose of making actual pictures of the rise and fall in volume of current in alternating circuits.
Fig. 3.--The "Duddell" Thermo-galvanometer. In this remarkable instrument alternating current enters at a, passes through the fine wire and leaves at b. In doing this it heats the wire, which in turn heats the lower end of the bismuth and antimony bars. This generates continuous current, which circulates through the loop of silver wire, c, which, since it hangs between the poles, d and e, of a magnet, is thereby turned more or less. The amount of the turning indicates the strength of the alternating current.
To realise the almost miraculous delicacy of these wonderful instruments we need first of all to construct a mental picture of what takes place in a circuit through which alternating current is passing. The current begins to flow: it gradually increases in volume until it reaches its maximum: then it begins to die away until it becomes nil: then it begins to grow in the opposite direction, increases to its maximum and dies away once more. That cycle of events occurs over and over again at the rate it may be of hundreds of times
per second. Now for the actual efficient operation of electrical machinery working on alternating current it is very necessary to know exactly how those changes take place--do they[40] occur gradually, the current growing and increasing in volume regularly and steadily, or irregularly in a jumpy manner? Engineers have a great fancy for setting out such changes in the form of diagrams, in which
13
case the alternations are represented by a wavy line, and it is of much importance to obtain an actual diagram showing not what the changes should be according to theory, but what they really are in practice. It is then possible to see whether the "wave-form" of the current is what it ought to be.
Once again we must turn our thoughts back to the string galvanometer. In that case, it will be remembered, there is a conducting fibre passing between the ends or poles of a powerful magnet, the result of which arrangement is that as the current passes through the fibre it is bent by the action of the magnetic forces produced around it. If the current pass one way, downwards let us say, the fibre will be bent one way, while if it pass upwards it will be bent the opposite way. Suppose then that we have two fibres instead of one, and that we send the current up one and down the other. One will be bent inwards and the other outwards. Then suppose that we fix a little mirror to the centre of the fibres, one side of it being attached to one fibre and the other to the other. As one fibre advances and the other recedes the mirror will be turned more or less. Consequently, as the current flowing in the fibres increases or decreases, or changes in direction, the mirror will be slewed round more or less in one direction or the other.
The spot of light thrown by the mirror will then dance from side to side with every variation, and if it be made to fall upon a rapidly moving strip of photograph paper a wavy line will be drawn upon the paper which will faithfully represent the changes in the current.
In its action, of course, it is not unlike an ordinary mirror galvanometer, but its special feature is in the mechanical arrangement of its parts which enable it to[41] move with sufficient rapidity to follow the rapidly succeeding changes which need to be investigated. It is far less sensitive than, say, a Thomson Galvanometer, but the latter could not respond quickly enough for this particular purpose.
[42]
CHAPTER III
THE FUEL OF THE FUTURE
We now enter for a while the realm of organic chemistry, a branch of knowledge which is of supreme interest, since it covers the matters of which our own bodies are constructed, the foods which we eat and the beverages which we drink, besides a host of other things of great value to us.
Although the old division of chemistry into inorganic and organic is still kept up as a matter of convenience, the old boundaries between the two have become largely obliterated. The distinction arose from the fact that there used to be (and are still to a very great extent) a number of highly complex substances the composition of which is known, for they can be analysed, or taken to pieces,
but which the wit of man has failed to put together. Consequently these substances could only be obtained from organic bodies. The living trees, or animals, could in some mysterious way bring these combinations about, but man could