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."
Fig. 1.-This shows the principle of this wonderful Galvanometer invented by Lord Kelvin in its latest form. Current enters at a, passes round the coils, as shown by the arrows, and away at b. A light rod, c, is suspended by the fine fibre, d, so that the eight little magnets hang in the centres of the coils—four in each. The current deflects these magnets and so turns the mirror, m, at the bottom of the rod. At e are two large magnets which give the little ones the necessary tendency to keep at "zero."
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.
Fig. 2.—Here we see the working parts of the "String Galvanometer," by which the beating of the heart can be registered electrically. The current flows down the fine silvered fibre, between the poles, a and b, of a powerful magnet. As the current varies, the fibre bends more or less.
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.