298 DEVELOPMENT OF HEAT. chap.
electrodes. In any case the mixture becomes exposed to such a high temperature, that the iron or copper fuses, and the alumina is reduced by the carbon to aluminium, which is taken up by the fused iron or copper. The electrodes are gradually drawn further and further apart, so that the current strength, read off on an interposed ammeter, remains approximately constant — about 5000 amperes are usually employed.
In the course of some hours the whole of the mixture will have undergone reaction; the furnace is then allowed to cool, and the melt withdrawn. One disadvantage of the process is that the activity of the furnace is inter- mittent, and consequently a good deal of heat is lost on cooling.
It has been stated that the Heroult process, which is in use amongst other places at the aluminium works at Neuhausen, is more economical than the Cowles' process which has been introduced at the works at Stoke-on- Trent.
In the Cowles* process direct currents can be used just as well as alternating currents without in any way inter- fering with the yield obtained from the current. This shows clearly that the electrolytic process plays no real part in the action, which depends only on the high temperature (essen- tial for the reduction of the alumina) attained by means of the electric current. In such cases alternating currents are to be preferred to direct ones, because then the process takes place uniformly at the two poles, and alternating currents of suitable electromotive force and strength can readily be obtained by the use of a transformer. If a polyphase current is employed, as many electrodes should 1)6 used as the current has phases, e.g. three with a three- pliase cmxent.
In the lighting of a Cowles' furnace we are reminded of the lighting of an arc lamp, and Maxim (12), in the furnace devised by him, has introduced lighting on the same principle as that made use of in Jablochkoff's electrical candle. Two
�� �
