After the cells have been in use for some time, it is a good plan to lift out the plates and remove all sediment which has settled to the bottom of the jars. A set of three such storage cells will have an E. M. F. of over six volts. Any number may be connected up in series in order to obtain a higher voltage. Storage batteries are usually rated in "ampere hours." An ampere hour is the amount of current represented by one ampere flowing for one hour. A ten-ampere-hour storage battery will deliver: One ampere for ten hours Two amperes for five hours Five amperes for two hours Ten amperes for one hour In other words, the result obtained by multiplying the number of amperes by the time in hours is the _ampere hour capacity_. A dynamo must have an E. M. F. of about ten volts in order to charge a three-cell storage battery. CHAPTER V ELECTRO-MAGNETISM AND MAGNETIC INDUCTION Connect two copper wires to a voltaic cell and stretch a portion of the wire over a compass needle, holding it parallel to it and as near as possible without touching. Then bring the free ends of the wires together and observe that the needle is deflected and after a few movements back and forth comes to rest at an angle with the wire. [Illustration: Fig. 76.—A Current of Electricity flowing through a Wire will deflect a Compass Needle.] Next form a rectangular loop of wire and place the needle within it as in Figure 77. A greater deflection will now be obtained. If a loop of several turns is formed, the deflection will be still greater. These experiments were first performed by Oersted, in 1819, and show that the region around a wire carrying a current of electricity has _magnetic_ properties. [Illustration: Fig. 77.—If a Loop of Wire is formed about a Compass Needle, the Deflection will be greater.] Another interesting experiment showing the magnetic effect of a current of electricity when passing through a wire may be performed by connecting a heavy copper wire to two or three bichromate-of-potash cells. Dip the wire into a pile of fine iron filings and a thick cluster of them will adhere to the wire as in Figure 78. As soon as the circuit is broken so that the current of electricity ceases flowing, the filings will fall off, showing that the magnetic effect ceases with the current. [Illustration: Fig. 78.—Iron Filings clustered on a Wire carrying a Current of Electricity.] These three simple experiments have shown that if a current of electricity is passed through a copper wire, the wire will deflect a compass needle, attract to itself iron filings, etc., as long as the current continues to flow. As soon as the current is shut off, the magnetic effect is _destroyed_. The region in the neighborhood of a wire carrying a current is a _field of force_ through which lines of magnetism are flowing in exactly the same way that they do in the neighborhood of a bar or horseshoe magnet.