. Dental electro-therapeutics. rve tissue or sensitive dentine a rheostatof this description is very essential in order to increase thecurrent without considerable discomfort. The most perfect rheostat for resistance of current fromvoltic-cell battery, useful for delicate work on the pulpsof teeth or obtaining sensitive tissue, is one of German RHEOSTATS 117 make, Fig. 47. It is constructed of a glass dial which turnson a central pivot, on the outer circle of the dial graphiteis impressed into the irregular etchings on the surface ofthe glass. This is graduated from very fine to coarserrecepta
. Dental electro-therapeutics. rve tissue or sensitive dentine a rheostatof this description is very essential in order to increase thecurrent without considerable discomfort. The most perfect rheostat for resistance of current fromvoltic-cell battery, useful for delicate work on the pulpsof teeth or obtaining sensitive tissue, is one of German RHEOSTATS 117 make, Fig. 47. It is constructed of a glass dial which turnson a central pivot, on the outer circle of the dial graphiteis impressed into the irregular etchings on the surface ofthe glass. This is graduated from very fine to coarserreceptacles for the graphite, in the manner shown in thefigure. The contact is made with mercury, which is letinto a slot underneath the dial, and acts as resistance. Byturning the glass dial over the mercury in contact withthe graphite conductor, the very minumum of currentstrength only is allowed to pass if the dial is slowly full resistance of this instrument is about 20,000 ohms,decreasing gradually to 20 ohms. Fig. 48. Wire resistance. A convenient and effective rheostat for a battery, whereno current collector is attached or necessary, consists ofa slate core around which is wound many hundred turnsof insulated wire, a sliding metallic contact spring is ad-justed to move over the coils of wire, making contact withportions of the wire from which the insulation has beenremoved, in such a manner that contact with each turn ofthe wire decreases the resistance by the amount of resistancein the length of wire that passes around the slate core,which, in this instance, is about volt, and the currentstrength is very gradually increased. This form of rheo-stat is largely used on switchboards to reduce the currentfrom the main. It requires careful attention in a dampclimate, for, should the wire oxidize, the insulation is liableto become imperfect, and the current brought on in irregular 118 ELECTRO-PHYSICS
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