_Measurement of Resistances._--When a resistance is to be measured that is within the range of the coils in _R__{1}, _R_ and _r_ are made equal. The needle of the galvanometer will move in a different direction, either to the right or to the left, according as the resistance in _R__{1} is greater or less than the line wire _x_. The needle remains at zero only when the resistance in _R__{1} is equal to that in _x_. For _r_ : _R_ :: _R__{1} : _x_. [Illustration: WHEATSTONE'S BRIDGE. PLATE XXVI] When the resistance of _x_ is greater than that of _R__{1}, as in an insulation test, the resistance in _r_ is made _less_ than that in _R_, in order that _r_ and _R_ may have such a proportion one to the other as will enable the coils in _R__{1} to balance a resistance in _x_, greater than their own, that is to say, greater than 11,100 ohms; thus _r_ : _R_ :: _R__{1} : _x_, or 10 : 1000 :: 10,000 : 1,000,000, the resistance in the line to be tested would be 1,000,000 ohms, supposing the values of _r_, _R_ and _R__{1} to be respectively 10, 1000, and 10,000 ohms. When the resistance to be tested is less than that of the least coil in _R__{1} (1 ohm), then the resistance in _r_ is made greater than in _R_. Thus _r_ : _R_ :: _R__{1} : _x_, or 100 : 10 :: 2 : 0·2; the resistance of the line to be tested would in this case be 1/20 of an ohm. _Manipulation._--In all cases the key in connection with the battery should first be depressed, then the galvanometer key, making very short contacts by the latter, just sufficient to show the direction of the deflection, until the coils in _R__{1} are nearly adjusted, otherwise considerable time will be lost in making a series of tests, owing to the swing given to the needle, which will take some little time before it again remains steady at zero. When once the coils in _R__{1} are adjusted, and a balance obtained, it should be ascertained whether the needle will remain steady when contact is made and broken. _Test Tables._--In connection with a system of testing electrical submarine mines, for the sake of convenience and simplicity it is necessary to use a table (termed a "Test Table"), on which all the apparatus used for the purpose of testing are fixed. Several forms of tables have been designed for such a purpose. At Fig. 95 is shown the method of arranging such a table.[L] _A_ is an astatic galvanometer placed between two switch plates, _B_ and _C_; ten other similar switch plates, 1, 2, 3, 4, _D_, 5, 6, 7, _E_, and 8, are arranged in front of the galvanometer _A_; _F_, _G_, and _H_ are three terminal plates; _K_ is a box of resistance coils used in connection with the thermo galvanometer _M_; _L_ is a firing key, and _N_ a battery commutator; _O_ is a three-coil galvanometer; _R_ is a Wheatstone balance (Post-office pattern).