. Electricity in diseases of the eye, ear, nose and throat . amount of voltage, or the effect upon the pat-ient is concerned. The resistance of a given conductor to a cur-rent is inversely proportional to the area of thecross section of the conductor and directly propor-tional to its length; hence, the combined resistanceof two conductors of equal area is one half of one ofthem. If any number of equal conductors be con-nected, separately or as one, with a source of vol-tage, they all receive the same amount of voltage butthe current depends upon the resistance. Let each of two conductors, havi
. Electricity in diseases of the eye, ear, nose and throat . amount of voltage, or the effect upon the pat-ient is concerned. The resistance of a given conductor to a cur-rent is inversely proportional to the area of thecross section of the conductor and directly propor-tional to its length; hence, the combined resistanceof two conductors of equal area is one half of one ofthem. If any number of equal conductors be con-nected, separately or as one, with a source of vol-tage, they all receive the same amount of voltage butthe current depends upon the resistance. Let each of two conductors, having 1000 ohmsresistance, be connected separately with a voltageof 100,—then each delivers a current of ,!°/?!*^ 1000 ohms ^:=1/10 of an ampere; or, combined, =i/5 A. If the conductors be joined in a parallel man-ner, they will have double the cross area of one andgive the same current as before: i. e. 500Ihms ~1/5 K. RHEOSTAT. 27 Coil Rheostat.—The principle of the usualcoil rheostat used in series with the patient, is readilydemonstrated in Figure -^^ xh= Pig. 19. A series rheostat, in which the current thatpasses from -f through A. to C. and through the pa-tient X, by means of the electrodes D. and E. to the— supply wire, is controlled by the switch C. Whenthe switch is placed at B., the resistance of the wholecoil, added to that of the patient, allows only a verylittle current to pass, C. is moved towards A. to les-sen the resistance in order to increase the current asrequired. Current Controller.—See Figure 20. In the figure, the current divides at A. passesfrom A. to B. through the coil and through theshunt circuit; that is, from A. through the patient C. to B. There is the same drop in voltage inthe coil between A. and C. as in the patients cir-cuit; hence, the nearer C. is to B. the greater thecurrent through the patient. The coil A. B. is graduated in volts, under the 28 PHYSICS OP ELECTEICITY. condition of its definite resistance, to show the drop
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