of the leaves it is very difficult to read accurately the amount of their movement and so to determine the charge which has been given to them.In a more recent improvement, therefore, only one strip of gold-leaf is used, the place of the other being taken by a copper strip. The whole of the movement is thus in the single gold-leaf, as the copper strip is comparatively stiff, and it is possible to arrange for the movement of this one piece of gold-leaf to be measured by a microscope.
The other principal kind of electrometer we owe, as we do the galvanometers, to the wonderful ingenuity of Lord Kelvin. In this the moving part is a strip of thin aluminium, which is suspended in a horizontal position by means,[35] generally, of a fine quartz fibre. Since it is necessary that this fibre should be a conductor, which quartz is not, it is electro-plated with silver. Thus a charge communicated to the upper end of the fibre, where it is attached to the case, passes down to the aluminium "needle," as it is called. Now the needle is free to swing to and fro, with a rotating motion, between two metal plates carefully insulated. Each plate is cut into four quadrants, the opposite ones being electrically connected, while all are insulated from their nearest neighbours. One set of quadrants is charged positively, and one set negatively, by a battery, but these charges have no effect upon the needle until it is itself charged.
As soon as that occurs, however, they pull it round, and the amount of its movement indicates the amount of the charge upon the needle, and therefore the pressure existing upon the charged body to which it is connected. The direction of its movement shows, moreover, whether the charge be positive or negative.
A little mirror is attached to the needle, so that its slightest motion is revealed by the movement of a spot of light, as in the case of the mirror galvanometers. Instruments such as these are called "Quadrant Electrometers."
My readers will remember, too, the "String Galvanometer" already mentioned. The same idea has been adapted to this purpose. A fine fibre is stretched between two charged conductors while the fibre is itself connected to the body whose charge is being measured. The charge which it derives from the body causes it to be deflected, which deflection is measured by a microscope.
In all cases of transmission of electricity over long distances for lighting or power purposes the currents are "alternating." They flow first one way and then the other, reversing perhaps twenty times a second, or it may be two hundred, or even more times in that
short period. Some electric railways are worked with alternating current, and it is used for lighting quite as much as direct current and is equally satisfactory.
[36]In wireless telegraphy it is essential. In that case, however, the reversals may take place millions of times per second. Consequently, to distinguish the comparatively slowly changing currents of a "frequency" or "periodicity" of a few hundreds per second from these much more rapid ones, the latter are more often spoken of as electrical oscillations. And these alternating and oscillating currents need to be measured just as the direct currents do. Yet in many cases the same instruments will not answer. There has therefore grown up a class of wonderful measuring instruments specially designed for this purpose, by which not only does the station engineer know what his alternating current dynamos are doing, but the wireless operator can tell what is happening in his apparatus, the investigator can probe the subtleties of the currents which he is working with, and apparatus for all purposes can be designed
and worked with a system and reason which would be impossible but for the possibility of being able to measure the behaviour of the subtle current under all conditions.
One trouble in connection with measuring these alternating currents is that they are very reluctant to pass through a coil.
One method by which this difficulty can be overcome has been mentioned incidentally already. I refer to the heating of a wire
through which current is passing. This is just the same whether the current be alternating or direct.
One of the simplest instruments of this class has been appropriated by the Germans, who have named it the "Reiss Electrical Thermometer," although it was really invented nearly a century ago by Sir William Snow Harris. It consists of a glass bulb on one end of
a glass tube. The current is passed through a fine wire inside this bulb, and as the wire becomes heated it expands the air inside the
bulb. This expansion moves a little globule of mercury which lies in the tube, and which forms the pointer or indicator by which
the instrument is read. As the temperature of the wire rises the mercury is forced away from the[37] bulb, as the temperature falls it returns. And as the temperature is varied by the passage of the current, so the movement of the mercury is a measure of the current.
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Another way is to employ a "Rectifier." This is a conductor which has the peculiar property of allowing current to pass one way but not the other. It thus eliminates every alternate current and changes the alternating current into a series of intermittent currents all in the same direction. Rectified current is thus hardly described by the term continuous, but still it is "continuous current" in the sense that the flow is always in the same direction, and so it can be measured by the ordinary continuous current instruments. The difficulty about it is that there is some doubt as to the relation between the quantity of rectified current which the galvanometer
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
