. Dental electro-therapeutics. probably owing tothe small amount of current—does not produce .T-raydermatitis. It is, however, of little use for radiographicwork owing to the length of exposure. Static currents possess therapeutic value in some cases. Primary Current.—This is now usually obtained fromthe main chiefly for these reasons: convenience, increasedvoltage, and quantity of current-accumulators requiringso much attention as well as being an expense. The supply from the main may be, however, continuous oralternating, and if the latter it must be rectified, i. e., madecontinuous. Continu
. Dental electro-therapeutics. probably owing tothe small amount of current—does not produce .T-raydermatitis. It is, however, of little use for radiographicwork owing to the length of exposure. Static currents possess therapeutic value in some cases. Primary Current.—This is now usually obtained fromthe main chiefly for these reasons: convenience, increasedvoltage, and quantity of current-accumulators requiringso much attention as well as being an expense. The supply from the main may be, however, continuous oralternating, and if the latter it must be rectified, i. e., madecontinuous. Continuous current in different parts of the country,unfortunately, may be supplied in various voltages. Alsoit is generated in some districts at as high voltage as480 volts, which is reduced for lighting and other purposesin mains of 240 volts by means of the three-wire system. THE X-RAYS OR RON THEN RAYS 161 This consists of having between the positive and negativewires a neutral wire, as shown by the following diagram. Fig. 74. Diagram of three-wire system. Now if current is taken from the mains A and B the volt-age will be 4S0. But if a neutral main wire C is used (elec-trically) between A and B, and connections made betweenA C and B C then will the current between A C and B C bereduced one-half—240 volts. It is also necessary that anequal current is available on both sides .when a large andsudden demand is made, as in switching on 20 or 30 current must be rectified, and probably themost satisfactory method is by means of a motor gener-ator, which consists of an alternating motor of the voltageof the main coupled to a continuous current dynamo ofthe requisite voltage and output in 162 ELECTRO-PHYSICS These are made so that the sudden switching on of alarge current to the coil is possible. A very efficient rectifier is the Nodon valve, and it ispossible to work a coil with it, but it is not so good as theabove. It is made of an aluminium rod and
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