Another system, and one that has been tried as a possible means of recording wireless messages, is as follows. The wireless arrangements consist of apparatus similar to that shown in Fig. 22, but instead of a Lodge coherer a Marconi valve is used, and an Einthoven galvanometer is substituted for the reflecting galvanometer. The Einthoven galvanometer consists of a very powerful electro-magnet, the pole pieces of which converge almost to points. A very fine silvered quartz thread is stretched between the pole pieces, as shown in Fig. 23, the tension being adjustable. The period of swing is about 1/250th of a second. A hole is bored through the poles, and one of them is fitted { 45} [Illustration] with a sliding tube which carries a short focus lens N. The light from M passes through the magnets, and a magnified image of the quartz thread is thrown upon the ebonite screen J. This screen is provided with a fine slit, and when the galvanometer is at rest the shadow of the thread just covers the slit in the screen and prevents any light from M reaching the photographic film. Upon signals being received the shadow of the thread moves to one side for a long or short period, uncovering the slit, and allowing light to pass through. The lens R concentrates the collected light to a point upon the revolving film. The connections for the complete receiver are given in Fig. 24.
The modified form of the Einthoven galvanometer, as arranged by Professor Korn for use with his selenium machines for photo-telegraphy over ordinary land lines, consists of two fine silver wires which are displaced in a lateral direction between the pole pieces when traversed by a current; the current passing through both wires in the same { 46} direction. A small shutter of aluminium foil is attached to the wires at the optical centre. The silver wires used are 1/1000 inch in diameter, with a natural period of about 1/120th of a second; the length of wires free to swing being usually about 5 cm.
The period of the wires depends to a great extent upon their length and diameter, and also upon their tension. By using short fine wires the period can be made much smaller, but a greater current is required to produce a similar displacement. Where the current available, as in wireless telegraphy, is very small, and a definite displacement of the wires is required, it is at once apparent that with wires of a given diameter there is a limit to their length and therefore to the period. Finer wires can be used, but here again there is a practical limit to their fineness, although galvanometers have been constructed with a single silvered quartz thread 1/12000th of an inch diameter, which, when placed in a powerful field, will give a good displacement with a current as small as 10^{ -8} ampere. { 47}