The detector used should be a Lodge wheel-coherer or a Marconi valve-receiver, as these are the only detectors that can be used with a recording instrument. If the swing of the galvanometer mirror is too great, a small battery with a regulating resistance can be inserted in order to limit the movement of the mirror to a very short range; the current of course flowing in an opposite direction to the current flowing through the coherer.
In this, as in all other methods of receiving, the results obtained depend upon the fineness of the line screen used in preparing the metal prints; and as already shown the fineness of the screen that can be used is dependent upon the mechanical efficiency of the entire apparatus.
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
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
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.