An electric picture can thus be made of the working of a man's heart. He holds in his hands two metal handles or is in some other way connected to the two ends of the fibre by wires just as the handles of a shocking coil are connected to the ends of the coil. The faint currents caused by the beating of his heart are thus set down in the form of a wavy line. Such a diagram is called a "cardiogram," and it seems that each of us has a particular form of cardiogram peculiar to himself, so that a man could almost be recognised and distinguished from his fellows by the electrical action of his heart. The galvanometer has a near relative, the electrometer, the astounding delicacy of which renders it equally interesting. It is particularly valuable in certain important investigations as to the nature and construction of atoms. The galvanometer, it will be remembered, measures minute currents; the electrometer measures minute pressures, particularly those of small electrically charged bodies. Every conductor (and all things are conductors, more or less) can be given a charge of electricity. Any insulated wire, for example, if connected to a battery will become charged--current will flow into it and there remain stationary. And that is what we mean by a charge as opposed to a current. Air compressed into a closed vessel is a charge. Air, however compressed, flowing along a pipe would be better described as a current. Imagine one of those cylinders used for the conveyance of gas under pressure and suppose that we desire to find the pressure of the gas with which it is charged. We connect a pressure-gauge to it, and see what the finger of the gauge has to say. What happens is that gas from the cylinder flows into the little vessel which constitutes the 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."