6. What will be the weight and resistance of 1,000 feet of No. 20 copper wire? 7. A storage battery sends 4 amperes of current through a plating solution. How much silver will it deposit in 2 hours? 8. (a) Compare the diameters of No. 22 and No. 16 copper wire. (b) Compare the lengths of the same wires giving 1 ohm resistance. (c) What relation exists between (a) and (b)? 9. Why is an electric bell circuit usually open while a telegraph line circuit is usually closed? 10. A copper wire and an iron wire of the same length are found to have the same resistance. Which is thicker? Why? 11. Why are electric bells usually arranged in parallel instead of in series? 12. What would happen if a voltmeter were put in series in a line? (3) OHM'S LAW AND ELECTRICAL CIRCUITS =271. Conditions Affecting Current Flow.=--Sometimes over a long circuit one cell will not work a telegraph sounder. In such a case, two, three, or more cells are connected so that the zinc of one is joined to the copper plate of the other. When connected in this way the cells are said to be _in series_ (Fig. 251). In the figure _A_ represents a voltmeter. It is found that _when cells are in series the E.M.F. of the battery is the sum of the electromotive forces of the cells_. An ammeter in the circuit shows increased current as the cells are added. Hence _if the resistance of the circuit remains unchanged, the greater the E.M.F. the greater is the current strength_. In this respect, the movement of electricity in a circuit is similar to the flow of water in a small pipe under pressure, as in the latter the flow of water increases as the pressure becomes greater. The current in a circuit may also be increased by lessening the resistance, since the current through a long wire is less than that through a short one, just as the flow of water will be greater through a short pipe than through a long one. To increase the current flowing in an electric circuit, one may therefore either increase the E.M.F. or decrease the resistance. [Illustration: FIG. 251.--Diagram of cells connected in series.] =272. Ohm's Law.=--The relation between the electromotive force applied to a circuit, its resistance, and the current produced was discovered in 1827 by George Ohm. Ohm's law, one of the most important laws of electricity, states that, in any circuit, _the current in amperes equals the electromotive force in volts divided by the resistance in ohms_. This principle is usually expressed thus: Current intensity = electromotive force/resistance or Amperes = volts/ohms or _I_ = _E_/_R_ [Illustration: FIG. 252.--The street cars are connected in parallel with each other.]