=134. Transmission of Currents.= It is often necessary to carry a current a long distance before it is used. A current of 50 amperes would need a copper conductor 25 times as large (sectional area) as one to carry the 10 ampere current mentioned in § 133. As copper conductors are very expensive, electric light companies, etc., generally try to carry the current on as small a wire as possible. To do this, the voltage is kept high, and the amperage low. Thus, as seen in § 133, the current of 1,000 volts and 10 amperes could be carried on a much smaller wire than the other current of equal energy. A current of 1,000 volts, however, is not adapted for lights, etc., so it has to be changed to lower voltage by some form of transformer before it can be used. =135. Transformers=, like induction coils, are instruments for changing the E. M. F. and strength of currents. There is very little loss of energy in well-made transformers. They consist of two coils of wire on one core; in fact, an induction coil may be considered a transformer, but in this a direct current has to be interrupted. If the secondary coil has 100 times as many turns of wire as the primary, a current of 100 volts can be taken from the secondary coil when the primary current is but 1 volt; but the _strength_ (amperes) of this new current will be but one-hundredth that of the primary current. By using the coil of fine wire as the primary, we can lower the voltage and increase the strength in the same proportion. [Illustration: Fig. 150.] [Illustration: Fig. 151.]