Fig. 179 shows lamps arranged side by side, or "in parallel," between the two main wires. The current divides, a part going through each lamp that operates. The total resistance of the circuit is not as large as in the series arrangement, as the current has many small paths in going from one main wire to the other. Fig. 179 also shows the ordinary _two-wire system_ for incandescent lighting, the two main wires having usually a difference of potential equal to 50 or 110 volts. These comparatively small pressures require fairly large conductors. _The Three-Wire System_, Fig. 180, uses the current from two dynamos, arranged with three main wires. While the total voltage is 220, one of the wires being neutral, 110 volts can be had for ordinary lamps. This voltage saves in the cost of conductors. [Illustration: Fig. 180.] [Illustration: Fig. 181.] _The Alternating System_, Fig. 181, uses transformers. The high potential of the current allows small main wires, from which branches can be run to the primary coil of the transformer. The secondary coil sends out an induced current of 50 or 110 volts, while that in the primary may be 1,000 to 10,000 volts. CHAPTER XX. HOW HEAT IS PRODUCED BY THE ELECTRIC CURRENT. =145. Resistance and Heat.= We have seen that all wires and conductors offer resistance to the electric current. The smaller the wire the greater its resistance. Whenever resistance is offered to the current, heat is produced. By proper appliances, the heat of resistance can be used to advantage for many commercial enterprises. Dynamos are used to generate the current for heating and lighting purposes. [Illustration: Fig. 182.] Fig. 182 shows how the current from two strong cells can be used to heat a short length of very fine platinum or German-silver wire. The copper conductors attached to the cells do not offer very much resistance. It will be seen from the above that in all electrical work the sizes of the wires used have to be such that they do not overheat. The coils of dynamos, motors, transformers, ampere-meters, etc., etc., become somewhat heated by the currents passing through them, great care being taken that they are properly designed and ventilated so that they will not burn out. [Illustration: Fig. 183.] [Illustration: Fig. 184.]