. Dental electro-therapeutics. s toone of the terminals of the battery, the other terminal is setparallel with the former and to these the conducting wiresare attached for using the current. The battery also is provided with a current reverser, avery necessary and convenient appliance for ionization. Awell-fitting cover keeps the battery free from dust, and adrawer in the side to keep electrodes and wires handy foruse. In using the battery all the cells should be switchedon and the current regulated through the rheostat, whichis finely graded and turns on the current very graduallyby sliding t
. Dental electro-therapeutics. s toone of the terminals of the battery, the other terminal is setparallel with the former and to these the conducting wiresare attached for using the current. The battery also is provided with a current reverser, avery necessary and convenient appliance for ionization. Awell-fitting cover keeps the battery free from dust, and adrawer in the side to keep electrodes and wires handy foruse. In using the battery all the cells should be switchedon and the current regulated through the rheostat, whichis finely graded and turns on the current very graduallyby sliding the contact shunt along the metal bar. A battery should not be used for ionization without arheostat, because the cell selector alone switches on too muchcurrent at a time, with the consequence that a painful shock BATTERIES 93 is experienced when each cell is added to the circuit. Mostof these batteries are made and sold by instrument makerswith only a collector, but a rheostat should be insisted on ifgood results are Fig. 32.—Galvanic battery. The current from a battery of 18 or 24 Leclanche cellsworks out according to Ohms law as follows: Presumingthe E. M. F. of each cell to be volts, which is very nearlycorrect, and the internal resistance (r) to be 3 ohms, with apatient in circuit of 1500 ohms resistance (R) the currentstrength (C) would be c = E. M. F. 18 cells X volts R +r R1500 + (r3 X 18) ampere = 17 ma. 94 MAGNETIC FIELD, CURRENTS AND BATTERIES Seventeen milliamperes is a great deal more current thanis required for ionization of the periodontal membrane, andthe E. M. F. from such a battery is ample. A battery of 24 cells on the same method of calculatingwill produce: r, E. M. F. ,.,,.-,., - 00 , C = = ? - = ampere = 22 ma. R + r 1500 + (3 X 24) The current from a voltaic cell battery is a continuousand smooth current, much more so than from any workingdynamo from which there is a certain amount of pulsation,although this pulsatio
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