In testing permanent joints special tests are carried out, which are described by Mr. Culley in his 'Handbook of Practical Telegraphy.' Voltaic batteries should be subjected to the following tests:-- 1.--For potential. 2.--For internal resistance. 3.--For electromotive force. For the purpose of testing the potential of a battery, one pole should be put to earth, and with the other one pair of the quadrants of a Thomson's reflecting galvanometer should be charged; when this is done, a certain deflection of the spot of light will occur, and the amount of such deflection, as compared with that produced by a standard cell applied to the instrument in a similar manner, would give the relative value of the potential of the battery. The following method of determining the internal resistance of a battery is that recommended by Mr. Latimer Clark in his book on electrical measurements. The instrument employed is a double shunt differential galvanometer, a diagram of which is shown at Fig. 97. Connect the battery and a set of resistance coils in circuit between the terminals _A_ and _D_, and insert plugs in the resistance coils so that they give no resistance; insert plugs at _A_ and _C_, and also both the shunt plugs at _A_ and _D_. The current will now flow through one half of the galvanometer circuit only, being, however, reduced to 1/100 of its amount by the shunt _D_; the deflection of the needle must be carefully read. The plug _A_ must now be removed to _B_, which causes the battery current to flow through both halves of the galvanometer (each being shunted). The circuit will now be as shown in the figure, and the needle will of course be deflected somewhat more than before. Now unplug the resistance coils which are in circuit with the battery until the deflection of the needle is reduced to its original amount, and the resistances unplugged will be equal to the internal resistance of the battery. The following is another method of ascertaining the internal resistance of a battery cell. A circuit is formed, consisting of the battery cell, a rheostat, and a galvanometer, and the strength _C_ is noted on the galvanometer. A second cell is then joined with the first, so as to form one of double the size, and therefore half the resistance, and then by adding a length _l_ of the rheostat, the strength is brought to what it originally was, _C_. Then if _E_ is the electromotive force, and R the resistance of cell, _r_ the resistance of the galvanometer, and other parts of the circuit, the strength _C_ in the one case is C = E / (R + _r_), and in the other = E / ((1/2)R + _r_ + _l_), and since the strength in both cases is the same, R = 2_l_, i.e., the internal resistance of the cell is equal to twice the resistance corresponding to the length _l_ of the rheostat wire.