For modern electrical machinery and apparatus copper is required of the utmost possible purity, for every impurity adds to its electrical resistance, in other words, diminishes its value as a conductor. Consequently thousands of tons of "electrolytic" copper, as it is termed, are produced every year. The electrodes used are plates of ordinary copper. A coating of pure metal is deposited by electrolysis upon the out-electrode from the other one. When the deposit is thick enough the out-electrode is taken out and the deposit torn off it, the union between the two being sufficiently imperfect for this to be done without difficulty. The metal of which the in-electrode is made has already been purified by other processes, until it contains but one per cent. of foreign matter, and by this means even that small percentage is entirely got rid of. The impurities fall to the bottom of the vessel in the form of "slime," which is periodically removed. And not only is electrolysis thus unerring in picking out certain atoms from among a mixture, but there is an exact relation between the work done and the quantity of current used. Consequently it forms a very exact method of measuring currents. The method of measuring current by the strength of the magnetic field which it produces has been mentioned already, and such measurements can be checked by electrolysis. Thus the practical definition of the ampere is "that current which when passed through a solution of silver nitrate in water will deposit silver at the rate of ·001118 gramme per second." The electric accumulator or secondary battery, one of the most useful appliances, is the result of electrolysis reversed. Many large electric-lighting plants have in addition to their generating machinery a large battery of secondary cells, which, being kept charged, are able to help the machinery in times of heavy demand, or even to supply the whole current needed for, say, half-an-hour, so that the whole of the machinery could, in the event of an accident, be shut down for that time and the supply maintained from the batteries. This would be sufficient in many cases for fresh machinery to be brought into action or emergency arrangements to be made. It may be that this book is being read by someone seated serenely in his arm-chair while engineers and workmen at the generating station are working in frantic haste to set right some sudden breakdown before the batteries are run down. The batteries may have saved the town half-an-hour's darkness. Large telegraph offices are fitted with secondary batteries. Many motorists owe the ignition which keeps their engines at work to secondary batteries. It is secondary batteries which keep the wireless apparatus at work on a wrecked vessel after the engines have stopped. Indeed secondary batteries are one of the most beneficent inventions. And if only they could be made in a lighter form than is possible at present their value would be infinitely increased.