=273. Resistance of Conductors in Series.=--A study of the _resistance_ of conductors when alone and when grouped in various ways is of importance _since, the current flow through any circuit is dependent upon its resistance_. The two most common methods of combining several conductors in a circuit are in _series_ and in _parallel_. Conductors are in _series_ when all of the current passes through each of the conductors in turn (Fig. 218), thus the cell, push-button, wires, and electric bell in an electric-bell circuit are in series. Conductors are in _parallel_ when they are so connected that they are side by side and a part of the whole current goes through each. None of the current that passes through one conductor can go through the conductors in parallel with it. Thus the electric street cars are in _parallel_ with each other. (See Fig. 252.) It is easily seen that none of the current passing through one car can go through any of the others. When the conductors are in _series_ the combined resistance is the _sum_ of the several resistances. Thus in an electric-bell circuit if the battery has a resistance of 1 ohm, the bell of 2 ohms, and the wire 1 ohm, the total resistance in the circuit is 4 ohms. When conductors are in _parallel_ the combined resistance is always _less_ than the separate resistances. Just as a crowd of people meets less resistance in leaving a building through several exits, so electricity finds less resistance in moving from one point to another along several parallel lines, than along one of the lines. =274. Resistance of Conductors in Parallel.=--If three conductors of equal resistance are in parallel, the combined resistance is just one-third the resistance of each separately (Fig. 253). The rule that states the relation between the combined resistance of conductors in parallel and the separate resistances is as follows:_The combined resistance of conductors in parallel is the reciprocal of the sum of the reciprocals of the several resistances_. For example, find the combined resistance of three unequal resistances in parallel; the first being 4 ohms, the second, 6 ohms, and third 3 ohms. The reciprocals of the three resistances are 1/4, 1/6, and 1/3. Their sum equals 6/24 + 4/24 + 8/24 = 18/24. The reciprocal of this is 24/18 which equals 1-1/3 ohms, the combined resistance. FIG. 253.--The three conductors are connected in parallel.