=310. Uses of Transformers.=--In electric lighting systems, dynamos often produce alternating currents at 1000 to 12,000 volts pressure. It is very dangerous to admit currents at this pressure into dwellings and business houses, so that transformers are installed just outside of buildings to "step-down" the high voltage currents to 110 or 220 volts. The lighting current that enters a house does not come directly from a dynamo. It is an induced current produced by a transformer placed near the house. (See Fig. 307.) In a perfect transformer the efficiency would be 100 per cent. This signifies that the energy that is sent into the primary coil of the transformer exactly equals the energy in the secondary coil. The best transformers actually show efficiencies better than 97 per cent. The lost energy appears as heat in the transformer. "The transfer of great power in a large transformer from one circuit to another circuit entirely separate and distinct, without any motion or noise and almost without loss, is one of the most wonderful phenomena under the control of man." =311. The mercury arc rectifier= is a device for changing an alternating current into a direct current. It is frequently used for charging storage batteries where only alternating current is supplied by the electric power company. It consists of an exhausted bulb containing two carbon or graphite electrodes marked _G_ in Fig. 308 and a mercury electrode marked _M_. It is found that current will pass through such a bulb only from the graphite to the mercury but not in the reverse direction. In operating the device, the secondary terminals of an alternating current transformer _T_ are connected to the graphite terminals of the rectifier. A wire connected to the center of the secondary of the transformer at _C_ is attached to the _negative_ terminal of the storage battery _SB_. The _positive_ terminal of the battery is connected to the mercury electrode of the rectifier tube through a reactance or choke coil _R_. This coil serves to sustain the arc between the alternations. _Sw_ is a starting switch, used only in striking the arc. It is opened immediately after the tube begins to glow. [Illustration: FIG. 308.--Diagram of a mercury arc rectifier.] Important Topics Transformer, induction coil, mercury arc rectifier, construction, action; uses of each. Exercises 1. Does the spark of an induction coil occur at "make" or at "break?" Why? 2. What must be the relative number of turns upon the primary and secondary coils of a transformer if it receives current at 220 volts and delivers current at 110? Also show by diagram. 3. Would the transformer work upon a direct current? Why? 4. Explain why the interrupter is a necessary part of the induction coil and not of the transformer. 5. If a building used eighty 110-volt incandescent lamps, what would be necessary to light them if they were joined in series? Why would this not be practical?