The terminal plates _G_ and _H_ are used for the connection, for testing purposes of the negative and positive poles, of the firing battery, and _F_ is connected with a zinc earth in the sea, for a similar purpose. These plates are in connection with the resistance coils _K_ and the thermo galvanometer _M_, employed for testing the firing battery, the circuit being closed by the firing key _L_. Other ways of using these plates may of course be adopted if desired. The resistance coils _K_ range from 0·5 to 100 ohms, and are composed of wire adapted for the passage of a quantity current. A reversing key is generally used in connection with a testing battery and the three-coil galvanometer _O_. This reversing key would consist of two bridges completely insulated from each other, the upper one attached to the negative, the lower one to the positive pole of the test battery. In their normal position both keys press against the upper bridge, and until one or other of the keys is pressed down no current will pass, the direction of the current being altered by pressing down a different key. The point of each key is provided with a terminal and connected, the one to a zinc earth through the switch plate 8, the other to one terminal of the three-coil galvanometer when the tests are to be applied. The Wheatstone balance _R_ is used in finding the resistances of electrical cables, balancing fuzes, &c. By means of a commutator, _N_, the necessary number of cells for any particular test may be thrown in circuit when required. _Test of Platinum Wire Fuze for Conductivity._--The platinum wire fuze may be tested electrically as follows:-- If placed in circuit with a few cells of a Daniell or Leclanché battery and a detector galvanometer, before the platinum wire bridge of the fuze is fixed, there should be no deflection of the needle, for no metallic circuit exists; if it did, such would be fatal to the efficiency of the fuze. If similarly placed in circuit after the bridge has been fixed, a considerable deflection of the needle should result, such deflection being due to the current passing through the metallic bridge, which to be efficient ought to be the sole medium through which the circuit is completed. _Test of Resistance of Platinum Wire Fuze._--The electrical resistance of a platinum wire fuze is ascertained by means of the Wheatstone's balance _R_ and galvanometer _A_, Fig. 95. The terminals of the fuze are connected to the binding screws of the balance, the commutator _N_ and galvanometer _A_ being connected up in circuit. The resistance of the coils is then adjusted by taking out plugs until the needle of the galvanometer _A_ is brought to zero, when the sum of the resistances indicated by the unplugged coils will be equal to that of the fuze. The resistance of a platinum wire fuze might also be ascertained by means of a differential galvanometer instead of a Wheatstone balance.