=268. Resistance of Circuits.=--Every part of an electrical circuit possesses resistance. In an electric-bell circuit, for instance, the wires, the bell, the push-button, and the cell itself, each offers a definite resistance to the passage of the current. The resistance _within the cell_ is termed _internal resistance_, while the resistance of the parts outside of the electric generator is called _external resistance_. =269. Electromotive Force.=--In order to set in motion anything, some _force_ must be applied. This is as true of electricity as of solids, liquids, or gases. By analogy that which is exerted by a battery or by a dynamo in causing current to flow is called an _electromotive force_. The unit of electromotive force, the _volt_, may be defined as _the electromotive force that will drive a current of 1 ampere through the resistance of 1 ohm_. The electromotive force of a dry cell is about 1.5 volts, of a Daniell cell 1.08 volts. Most electric light circuits in buildings carry current at 110 or 220 volts pressure. Currents for street cars have an electromotive force of from 550 to 660 volts. [Illustration: FIG. 249.--Diagram of a commercial voltmeter.] =270. The Voltmeter.=--An instrument for measuring the electromotive force of electric currents is called a _voltmeter_ (Fig. 249). It is usually a moving-coil galvanometer, and is always of _high resistance_. It is like an ammeter in construction and appearance. In fact, a voltmeter is an ammeter which has had its shunt removed or disconnected. In place of a shunt, the voltmeter uses a coil of wire of high resistance (see _R_, Fig. 249) _in series_ with the galvanometer coil. The high resistance of the voltmeter permits but a very small current to flow through it. Hence a voltmeter must be placed _across_ a circuit and not in it. In other words _a voltmeter is connected in shunt_, while _an ammeter is in series with the circuit_ as is shown in Fig. 250. DIMENSIONS AND FUNCTIONS OF COPPER WIRES Column headings: B: B. & S. gauge number MM: Millimeters SA: Sectional area in square millimeters WL: Weight and length, Density = 8.9, feet per pound R: Resistance at 24°C., feet per ohm C: Capacity in amperes