ceased the untwisting of the silk brought it back to its original position again.
So far it seems to differ very little from the ordinary galvanometer previously mentioned, but the stroke of genius was in the method of reading it. With a small current the movement of the magnet was too small to be observed by the unaided eye, so it was attached to a minute mirror made of one of those little circles of glass used for covering microscope slides, silvered on the back. The magnet was cemented to the back of this, yet both were so small that together their weight was supported by a single thread of cocoon silk. Light from a lamp was made to fall upon this mirror, thereby throwing a spot of light upon a distant screen. Thus the slightest movement of the magnet was magnified into a considerable movement of the spot of light. The beam of light from the mirror to the screen became, in fact, a long lever or pointer, without weight and without friction.
The task of watching the rocking to and fro of the spot of light was found to be too nerve-racking for the telegraph operators, and so Lord Kelvin improved upon his galvanometer in two ways. He first of all managed to give it greater turning-power, so that, actuated by the same current, the new instrument would work much more strongly than the older one. Then he utilised this added power to move a pen whereby the signals were recorded automatically upon a piece of paper. The new instrument is known as the Siphon Recorder.
The added power was obtained by turning the instrument inside out, as it were, making the coil the moving part and the permanent magnet the fixed part. This enabled him to employ a very powerful permanent magnet in place of the minute one made of watch-spring. The interaction of two magnets is the result of their combined strength, and that of the coil being limited by the strength of the minute current the only way to increase the combined power of the two was to substitute a large powerful magnet for the small magnetised watch-spring. This large magnet would, of course, have been too heavy to swing easily and therefore the positions had to be reversed.
So now we have two types of galvanometer, both due originally to the inventions of Lord Kelvin. For some purposes the Thomson type (his name was Thomson before he became Lord Kelvin) are still used, but in a slightly elaborated form. Its sensitiveness is such that a current of a thousandth of a micro-ampere will move the spot of light appreciably. And when one comes to consider that a micro-ampere is a millionth part of an ampere this is perfectly astounding.
But there is a more wonderful story still to come, of an instrument which can detect a millionth of a micro-ampere, or one millionth of a millionth of an ampere. It is not generally known that we are all possessors of an electric generator in the form of the human heart, but it is so, and Professor Einthoven, of Leyden, wishing to investigate these currents from the heart, found himself in need of a galvanometer exceeding in sensitiveness anything then known. Even the tiny needles or coils with their minute mirrors have some weight and so possess in an appreciable degree the property of inertia, in virtue of which they are loath to start movement and, having started, are reluctant to stop. This inertia, it is easy to see, militates against the accurate recording of rapid variations in minute currents, so the energetic Professor set about devising a new galvanometer which should answer his purpose. This is known as the "String Galvanometer."
The main body of the instrument is a large, powerful electro-magnet, in shape like a large pair of jaws nearly shut. Energised by a strong current, this magnet produces an exceedingly strong magnetic field in the small space between the "teeth" as it were. In this space there is stretched a fine thread of quartz which is almost perfectly elastic. It is a non-conductor, however, so it is covered with a fine coating of silver. Silver wire is sometimes used, but no way has yet been found of drawing any metallic wire so thin as the quartz fibre, which is sometimes as thin as two thousandths of a millimetre, or about a twelve-thousandth of an inch. A hundred pages of this book make up a thickness of about an inch, so that one leaf is about a fiftieth of an inch. Consequently the fibre in question could be multiplied 240 times before it became as stout as the paper on which these words are printed.
The current to be measured, then, is passed through the stretched fibre and the interaction of the magnetic field by which the fibre is then surrounded, with the magnetic field in which it is immersed, causes it to be deflected to one side. Of course the deflection is exceedingly small in amount, and as it is undesirable to hamper its movements by the weight of a mirror, no matter how small, some other means of reading the instrument had to be devised. This is a microscope which is fixed to one of the jaws, through a fine hole in which the movements of the fibre can be viewed. Or what is often better still, a picture of the wire can be projected through the microscope on to a screen or on to a moving photographic plate or strip of photographic paper. In the latter case a permanent record is made of the changes in the flowing current.
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
