WIRING UP A HOUSE.--The system of wiring up a house so that all doors and windows will be connected to form a burglar alarm outfit, is shown in Fig. 61. It will be understood that, in practice, the bell is mounted on or at the annunciator, and that, for convenience, the annunciator box has also a receptacle for the battery. The circuiting is shown diagramatically, as it is called, so as fully to explain how the lines are run. Two windows and a door are connected up with an annunciator having three drops, or numbers 1, 2, 3. The circuit runs from one pole of the battery to the bell and then to one post of the annunciator. From the other post a wire runs to one terminal of the switch at the door or window. The other switch terminal has a wire running to the other pole of the battery. A, B, C represent the circuit wires from the terminals of the window and door switches, to the annunciators. It is entirely immaterial which side of the battery is connected up with the bell. From the foregoing it will readily be understood how to connect up any ordinary apparatus, remembering that in all cases the magnet must be brought into the electric circuit. CHAPTER VIII ACCUMULATORS. STORAGE OR SECONDARY BATTERIES STORING UP ELECTRICITY.--In the foregoing chapters we have seen that, originally, electricity was confined in a bottle, called the Leyden jar, from which it was wholly discharged at a single impulse, as soon as it was connected up by external means. Later the primary battery and the dynamo were invented to generate a constant current, and after these came the second form of storing electricity, called the storage or secondary battery, and later still recognized as accumulators. THE ACCUMULATOR.--The term _accumulator_ is, strictly speaking, the more nearly correct, as electricity is, in reality, "_stored_" in an accumulator. But when an accumulator is charged by a current of electricity, a chemical change is gradually produced in the active element of which the accumulator is made. This change or decomposition continues so long as the charging current is on. When the accumulator is disconnected from the charging battery or dynamo, and its terminals are connected up with a lighting system, or with a motor, for instance, a reverse process is set up, or the particles re-form themselves into their original compositions, which causes a current to flow in a direction opposite to that of the charging current. It is immaterial to the purposes of this chapter, as to the charging source, whether it be by batteries or dynamos; the same principles will apply in either case. [Illustration: _Fig. 62._ ACCUMULATOR GRIDS] ACCUMULATOR PLATES.--The elements used for accumulator plates are red lead for the positive plates, and precipitated lead, or the well-known litharge, for the negative plates. Experience has shown that the best way to hold this material is by means of lead grids.