A manual of practical medical electricity - the Röntgen rays and Finsen light (1902) (14780511601)

Identifier: manualofpractica00turn (find matches)

Title: A manual of practical medical electricity : the Röntgen rays and Finsen light

Year: 1902 (1900s)

Authors: Turner, Dawson

Subjects: X-Rays Electrophysiology Electrosurgery Electric Stimulation Therapy Electrotherapeutics X-rays Electrophysiology Electrosurgery

Publisher: New York : William Wood & Company

Contributing Library: Francis A. Countway Library of Medicine

Digitizing Sponsor: Open Knowledge Commons and Harvard Medical School

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d external,to amount to one hundred ohms, and the current-strengthto ten milliamperes ; then 100 ohms X01 ampere = E. M. F. = 1 volt. Further, the greater the external resistance, the less weneed consider the internal in making our calculation ; thus,if we can put a resistance of 2,000 ohms into the externalcircuit, yielding us a current of, say, one milliampere, it willmatter but little whether in the case of a cell with aninternal resistance of five ohms we add this on or not, for 2,000 ohms X 001 ampere = 2 volts, and2,005 ohms X 001 ampere = 2*005 volts. A Daniell cell can be used as a standard of comparison.It has an E. M. F. of one volt; then, as the current- Elfxtromotive Force 57 strength varies directly as the E. M. F., and inversely asthe resistance, all we need do is to pass the current fromour cells, and from the Daniell, through a milliamperemeter with a definite high resistance in the circuit. The -83 §^ \ i<^^ •1 ^ S rm ■A 1 ,=J 15 » O 11 H . 1— ir. --- -;^ O

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amount of this resistance makes no difference ; it need notbe known. All that is necessary is that precisely the sameresistance should be used in the two cases, and the higherthe resistance is, the less attention we need pay to the 58 A Manual of Practical Medical Electricity internal resistances of the cells. Suppose we have a resist-ance of about 500 ohms, and that our cells give us acurrent of 20 m.a., and the Daniell a current of 2 m.a. ;then 2 : 20 : : I : %=io volts. Further, if we have a means of throwing in a resistanceof exactly t,ooo ohms, including the resistance of the gal-vanometer, and the latter be graduated in milliamperes,then we convert our galvanometer into a voltmeter, read-ing volts for milliamperes ; for by Ohms law one volt in acircuit of 1,000 ohms yields a current of one milliampere,and ten volts ten milliamperes, and so on. The galvano-meters of Dr. Edelmann can be purchased with thisarrangement—for medical men probably the most con-venient. E. M. F. may be

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1902

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