These wires are joined to the extremities of the nickel wires in the receiver-box and then, passing through the lead pipe, they enter another metal box which contains a battery and electric bell. The pinch of nickel filings in the small ebonite box is not an electric conductor in its ordinary condition, and hence the electric circuit, including the battery and bell, is not complete. If, however, an electric oscillation is set up in the nickel receiver-wires, the mass of metal particles connecting them at once becomes a conductor, because little metallic granules stick or cohere together. The battery is thus able to send an electric current through the circuit, which includes the coherer, and the electric bell is caused to ring. It may be mentioned that in the actual apparatus employed the arrangement is not quite so simple. The coherer would be permanently injured if we were to attempt to send through it an electric current strong enough to ring an electric bell. Hence we associate with the coherer a contrivance called a _relay_. A single voltaic cell, E (a dry cell) (see Fig. 75), is joined up in series with the coherer C and this relay R. The latter is a sort of switch or circuit-closer of such kind that when a very feeble current passes through it it closes a second circuit through which a much stronger current can pass. The transition of the nickel filings from a non-conductive to a conductive condition is, therefore, only the means by which a very small current of electricity is allowed to pass through the circuit of an electro-magnet which forms the circuit of the relay. This action causes a piece of iron to be attracted, and this again in turn closes another circuit, and so enables the current from a second battery, F, of five or six cells to actuate the electric bell G. The arrangement of the two batteries, the relay coherer, and bell will be understood by studying the diagram of connections in Fig. 75. The really important condition in securing success in the performance of the experiments made with this apparatus is that the long wires which connect the receiver-box with the metal box containing the bell, battery, and relay shall be entirely enclosed in a lead pipe without joint, which is soldered at one end into the receiver-box and at the other into the battery-box. Another practical point is that these wires, where they enter the battery-box, must have included in their circuit two little coils of insulated wire of a good many turns, which are called “choking coils.” A third element of success is that the coherer or sensitive conductor shall be sensitive enough, but not too sensitive. This condition can only be obtained by a process of trial and failure. Being provided with these two pieces of apparatus, we can now proceed to exhibit a series of experiments of great interest. [Illustration: FIG. 75.]