gauge and there records its own pressure.
And just the same applies with electrometers. Precisely as the pressure-gauge measures the pressure of air or gas in some vessel, so the electrometer measures the electrical pressure in a charged body.
Further, some of the charged bodies with which the student of physics is much concerned are far smaller than can be seen with the most powerful microscope. How wonderfully minute and delicate, therefore, must be the instrument which can be influenced by the tiny charge which so small a body can carry.
It will be interesting here to describe an experiment performed with an electrometer by Professor Rutherford, by which he determined how many molecules there are in a centimetre of gas, a number very important to know but very difficult to ascertain, since molecules are too small to be seen. This number, by the way, is known to science as "Avogadro's Constant."
Everyone has heard of radium, and knows that it is in a state which can best be described as a long-drawn-out explosion. It is always shooting off tiny particles. Night and day, year in and year out, it is firing off these exceedingly minute projectiles, of which there are two kinds, one of which appears to be atoms of helium.
Some years ago, when radium was being much talked about and the names of M. and Madame Curie were in everyone's mouth, little toys were sold, the invention, I believe, of Sir William Crookes, called spinthariscopes. Each of these consisted of a short brass tube with a small lens in one end. Looking through the lens in a dark room, one could see little splashes of light on the walls of the tube. Those splashes were caused by a tiny speck of radium in the middle of the tube, the helium atoms from which, by bombarding the inner surface of the tube, produced the sparks.
Now if we can count those splashes we can tell how many atoms of helium are being given off per minute. And if then we reckon how many minutes it takes to accumulate a cubic centimetre of helium we can easily reckon how many atoms go to the cubic centimetre. But the difficulty is to count them.
So the learned Professor called in the aid of the electrometer. He could not count all the atoms shot off, so he put the piece of radium at one end of a tube and an electrometer at the other. Every now and then an atom shot right through the tube and out at the farther end. And since each of these atoms from radium is charged with electricity, each as it emerged operated the electrometer. By simply watching the twitching of the instrument, therefore, it was possible to count how many atoms shot through the tube—one atom one twitch. And from the size and position of the tube it was possible to reckon what proportion of the whole number shot off would pass that way.
The result of the experiment showed that there are in a cubic centimetre of helium a number of atoms represented by 256 followed by seventeen noughts. And as helium is one of the few substances in which the molecule is formed of but one atom, that is also the number of molecules.
And now consider this, please. A cubic centimetre is about the size of a boy's marble. That contains the vast number of molecules just mentioned. And the electrometer was able to detect the presence of those one at a time. Need one add another word as to the inconceivable delicacy of the instrument.
In its simplest form the electrometer is called the "electroscope." Two strips of gold-leaf are suspended by their ends under a glass or metal shade. As they hang normally they are in close proximity. Their upper ends are, in fact, in contact and are attached to a small vertical conductor. A charge imparted to the small conductor will pass down into the leaves, and since it will charge them both they will repel each other so that their lower ends will swing apart. Such an instrument is very delicate, but because of the extreme thinness 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, 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.
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
