. Medical electricity; a practical treatise on the applications of electricity to medicine and surgery. may be composed. Bymeans of a selector any possible number of cups from oneup to the limit of the battery may be selected for the desiredapplication. The pole-board should also contain a polaritychanger, an arrangement for quickly changing the poles,a commutator, and an interrupter (rheotome), whichmay run either by clock-work or by an electro-magnet, forinterrupting the current slowly or quickly as may be neces-sary. Besides these, pole-boards are usually supplied witha galvanometer, a rheo
. Medical electricity; a practical treatise on the applications of electricity to medicine and surgery. may be composed. Bymeans of a selector any possible number of cups from oneup to the limit of the battery may be selected for the desiredapplication. The pole-board should also contain a polaritychanger, an arrangement for quickly changing the poles,a commutator, and an interrupter (rheotome), whichmay run either by clock-work or by an electro-magnet, forinterrupting the current slowly or quickly as may be neces-sary. Besides these, pole-boards are usually supplied witha galvanometer, a rheostat, or resistance coils. The ordi-nary galvanometer, which, theoretically, measures the forceof the current, does not actually afford constant and relia-ble indications, and can be depended on only to indicatethe direction of the current. The cheek and tongue of theoperator become in actual practice a delicate and trust-worthy galvanometer; but now no pole-board can be con-sidered properly equipped which does not contain a gal-vanometer (a niilliamperemcter~) graduated in absolute units. Fig. Resistance coils. Most pole-boards are supplied with a water rheostat bywhich different degrees of resistance are brought withinthe circuit, but for any nice determination resistance coilsare necessary. As the resistance offered by a wire dependson its length and the area of its section, it is obvious that, FORMS OF GALVANIC COMBINATIONS. 61 by having coils of certain standards of length and thick-ness, a fixed and definite amount of resistance can be in-troduced into the circuit. Such are the resistance coilsnow used to be interpolated in the circuit, for inducingdefinite resistances in ohms (Fig. 30). The Simens unitof resistance consists of mercury having the temperatureof o° C, contained in a glass tube one metre long and one Fig. 31.
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectelectro, bookyear1887
