With all the plugs inserted, the electrical current will flow direct from _T_ to _T__{1}, the large metallic junction pieces directly connected by the plugs would offer no sensible resistance; but if all the plugs were removed, then the current would flow through each of the coils 1, 2, 3, and 4, and the resistance in the circuit would be the sum of the resistances of those four coils. With the plugs arranged as in the figure, the current would flow through coil 4 only, and the resistance in the circuit would be equal to the resistance of that coil. _Wheatstone's Balance._--The electrical conductivity of a body is determined by ascertaining the ratio between the resistance of a certain length of the conductor in question, having a given section, to that of a known length of a known section of some substance taken as a standard. For this purpose Wheatstone's bridge in connection with a box of resistance coils is the most convenient method. At Fig. 94 is shown Wheatstone's balance (Post-office pattern), and at Fig. 93 the apparatus is reduced into the form of a parallelogram, which is the usual diagram of Wheatstone's bridge. The theory of the bridge is as follows: Four conductors _A B_, _B C_, _A D_, and _D C_ are joined at _A_ and _C_ to the poles of a battery _Z_; the resistance between _A_ and _B_ is _R_; that between _A_ and _D_ is _r_; that between _D_ and _C_ is _R__{1}; and that between _B_ and _C_ is _x_, the unknown resistance to be measured. A convenient constant ratio is chosen for _R__{1} and _r_, such as equality 1 to 10, 1 to 100, or 1 to 1000; and then _R__{1} is adjusted until no current flows through the galvanometer _G_; when this is the case we have R : _r_=R_{1} : _x_, or _x_ = (_r_/R) × R_{1}; so that if _r_ = R/100, _x_ will be equal to R_{1}/100. Two keys _a_ and _b_ are inserted; the current is wholly cut off the four conductors until contact is made at _a_; and then after the currents in the four conductors have come to their permanent condition, contact is made at _b_ to test whether any current flows through the galvanometer. The three resistances _R_, _R__{1} and _r_ and the resistance of the galvanometer should be small if _x_ is small, and great if _x_ is great. The conductors _A B_ and _A D_ of the bridge are each formed of three resistance coils having a resistance of 10, 100, and 1000 ohms respectively, inserted between the terminals _B_ and _D_ of the balance, Fig. 94. The conductor _D C_ is formed of a set of resistance coils from 1 up to 4000 ohms, amounting altogether to 11,110 ohms, inserted between the terminals _D_ and _C_ of the balance; in the balance, a brass plug being inserted between the terminals _D_ and _D__{1}, they may be considered as one terminal _D_. The conductor _B C_ is the wire to be tested, and is connected to the terminals _B_ and _C_ of the balance.