This rule may be understood better if we consider the _conductance_ of the conductors in parallel. Since the conductance of a two ohm wire is just one-half that of a one-ohm wire, we say that the conductance of a body is inversely as the resistance, or that it is the _reciprocal of the resistance_. The conductance of the 4-, 6-, and 3-ohm coils will therefore be respectively 1/4, 1/6, and 1/3, and since the combined conductance is the sum of the several conductances, the total conductance is 18/24. Also since this is the reciprocal of the total resistance, the latter is 24/18 or 1-1/3 ohms. When two or more conductors are connected in parallel each one is said to be a _shunt_ of the others. Many circuits are connected in _shunt_ or in parallel. Fig. 254 represents four lamps in parallel. Incandescent lamps in buildings are usually connected in parallel, while arc lamps are usually connected in series. Fig. 255 represents four lamps in series. Important Topics 1. Conditions affecting current flow, (a) E.M.F., (b) resistance. 2. Ohm's law, three forms for formula. 3. Resistance of conductors: (a) in series, (b) in parallel; how computed, illustrations. [Illustration: FIG. 254.--The four lamps are connected in parallel.] [Illustration: FIG. 255.--The four lamps are connected in series.] Exercises 1. What current flows through a circuit if its E.M.F. is 110 volts and the resistance is 220 ohms? 2. A circuit contains four conductors in series with resistances of 10, 15, 6, and 9 ohms respectively. What current will flow through this circuit at 110 volts pressure? What will be the resistance of these four conductors in parallel? 3. What is the combined resistance of 8 conductors in parallel if each is 220 ohms? What current will flow through these 8 conductors at 110 volts pressure? 4. What is the resistance of a circuit carrying 22 amperes, if the E.M.F. is 20 volts? 5. What E.M.F. will send 8 amperes of current through a circuit of 75 ohms resistance? 6. How does the voltmeter differ from the ammeter? 7. How can one determine the resistance of a conductor? 8. The resistance of a hot incandescent lamp is 100 ohms. The current used is 1.1 amperes. Find the E.M.F. applied. 9. What is the resistance of the wires in an electric heater if the current used is 10 amperes, the voltage being 110? 10. The resistance of 1000 ft. of No. 36 copper wire is 424 ohms. How many feet should be used in winding a 200 ohms relay? 11. The resistance of No. 00 trolley wire is 0.80 ohm per 1000 ft. What is the resistance of a line 1 mile long? 12. A wire has a resistance of 20 ohms. It is joined in parallel with another wire of 6 ohms, find their combined resistance. 13. The separate resistances of two incandescent lamps are 200 ohms and 70 ohms. What is their combined resistance when joined in parallel? When joined in series? (4) METHODS OF GROUPING CELLS AND MEASURING RESISTANCE