receiver shall be so arranged that it can work up and accumulate the impulses of the radiator, and before attaining its maximum swing can overflow into the coherer associated with it and thus give the signal. [Illustration: FIG. 26 (Fig. 10 of Specification 11,575/97).--Interchangeable Self-Induction Coils for signalling to different stations.] [Illustration: FIG. 27 (Fig. 3 of Specification 11,575/97).--Diagrammatic representation of Syntonic Radiator and Receiver. The middle spark gap _h_₂ _h_₃ is unnecessary, though sometimes helpful. The main charging is done by impulsive rush at the outside knobs.] [Illustration: FIG. 28 (Fig. 7 of Specification 11,575/97).--Syntonic Radiator with earth connection arranged for sending.] The general appearance of a pair of signalling stations on this plan is shown in Fig. 24, where the huts contain the sending and receiving instruments. The self-induction coil joining the two capacity-areas is better depicted in Fig. 25, which also shows one mode of joining up the coherer to a syntonic receiver. (The galvanometer and shunt are, of course, merely typical of any kind of telegraphic instrument whatever.) Fig. 26 indicates one form of sender with three alternative syntonising coils for speaking to three distant attuned stations. Fig. 27 shows a radiator arranged for receiving, but illustrates another method of charging, and one frequently employed by the author, viz., the method by impulsive rush (compare Figs. 11, 12 and 19, on pp. 14 and 25 of this book). The terminals of the Ruhmkorff coil are here connected, not to the capacity-areas direct, but to a pair of knobs near the centre of gravity of each area, so that when the discharge occurs each area is suddenly charged oppositely, and the two opposite charges are left to surge into one another and set up the oscillations. This impulsive method of charging is essentially that adopted in the spherical whip-crack emitter depicted in Fig. 19 (p. 25, _ante_), the two poles of the sphere having but small capacity and being joined by as thick a conductor as the equator of the sphere. But for such a radiator as is indicated in Fig. 24 or Fig. 27 the author commonly found that a third short spark gap in the middle was an improvement, and so, as is well known, did Prof. Righi find it, and embodied it in his well-known double-sphere double-knob emitter. [Illustration: FIG. 29 (Fig. 12 of Specification 11,575/97).--Single-point Coherer, with clockwork Tapper-back operating on the projecting end of the spring clamped at P and lightly touching a needle point _n_.]