Page:Heroes of the telegraph (IA cu31924031222494).djvu/108

 The principle of the siphon recorder is exactly the inverse of the mirror galvanometer. In the latter we have a small magnet suspended in the centre of a large coil of wire—the wire enclosing the magnet, which is free to rotate round its own axis. In the former we have a small coil suspended between the poles of a large magnet—the magnet enclosing the coil, which is also free to rotate round its own axis. When a current passes through this coil, so suspended in the highly magnetic space between the poles of the magnet, the coil itself experiences a mechanical force, causing it to take up a particular position, which varies with the nature of the current, and the siphon which is attached to it faithfully figures its motion on the running paper.

The point of the siphon does not touch the paper, although it is very close. It would impede the motion of the coil if it did. But the 'capillary attraction' of so fine a tube will not permit the ink to flow freely of itself, so the inventor, true to his instincts, again called in the aid of electricity, and electrified the ink. The siphon and reservoir are together supported by an ebonite bracket, separate from the rest of the instrument, and insulated from it; that is to say, electricity cannot escape from them to the instrument. The ink may, therefore, be electrified to an exalted state, or high potential as it is called, while the body of the instrument, including the paper and metal writing-tablet, are in connection with the earth, and at low potential, or none at all, for the potential of the earth is in general taken as zero.

The ink, for example, is like a highly-charged thunder-cloud supported over the earth's surface. Now the tendency of a charged body is to move from a place of higher to a place of lower potential, and consequently the ink tends to flow downwards to the