Two forms of electric light are used - the arc and the incandescent. When a strong current passes between two carbon points which have been first in contact and are afterwards separated a short distance, the interval between them is occupied by a luminous band (known as the voltaic arc). This constitutes the arc light. Its light is very intense. The common street light is equal to about 800 candle-power, but the powerful search light may equal more than 50,000,000 candles. In the incandescent lamp the current is made to pass through a strip of carbon which has been carefully prepared and formed into a loop. This becomes intensely hot, and gives out much light. It is enclosed in a small glass globe, from which the air must be removed or a vacuum formed, otherwise the heated carbon would unite with the oxygen and the air be consumed. This light is suitable for household use. The current is supplied by an electro-magnetic machine.