. Dental electro-therapeutics. is a pilot lamp at thetop of the board which indicates the presence of the current strength of the different instruments is regu-lated by the length, thickness, and number of coils in thewire which form the rheostats at the back of the board,different amperage being necessary for different individualinstruments or sets of instruments. When the current is switched on, a large amount of cur-rent is constantly passing through the shunt circuit whichof course is not used, in this there is considerable waste, ALTERNATING CURRENT TRANSFORMER 129 and the cur
. Dental electro-therapeutics. is a pilot lamp at thetop of the board which indicates the presence of the current strength of the different instruments is regu-lated by the length, thickness, and number of coils in thewire which form the rheostats at the back of the board,different amperage being necessary for different individualinstruments or sets of instruments. When the current is switched on, a large amount of cur-rent is constantly passing through the shunt circuit whichof course is not used, in this there is considerable waste, ALTERNATING CURRENT TRANSFORMER 129 and the current should not be left on the switchboardexcept it is in use. An illustration of a switchboard forheavy currents appears on p. 149. Alternating Current Transformers.—It is desirable some-times to change the alternating current from the maininto continuous current, or to transform it into one of lowvoltage and high amperage for cautery or lamp; this isaccomplished by induction, or motor transformers or byrectifiers. Fig. 01. Transformer for light and cauter By an induction coil on the principle already mentioned,the current of high voltage can be transformed into oneof low E. M. F. and high current strength, or by havingtwo coils wound on a ring of soft iron, a primary with agreater number of turns in the coil than the secondary, thecurrent passes through the primary and induces currentin the secondary which is in the magnetic field of the transformed in this manner is used for cauteryand light. For producing the opposite effect, that is, trans-forming the current into much higher E. M. F., the winding9 130 ELECTRO-PHYSICS of the coils is reversed, the primary with a fewer numberto turns in the coil than the secondary. Current trans-formed in this manner can be used for high frequencyapparatus, or if a synchronous commutator in the secondarycircuit is employed, for the x-ray work. The switchboardsare provided with sliding resistance for adjusting the volt-age to
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