A manual of practical medical electricity - the Röntgen rays and Finsen light (1902) (14781304054)

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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parallel, and thenthese two groups in series ; that would be equivalent toconnecting two cells, each five times as large as a singlecell, in series. The E. M. F. would therefore be that of twocells, viz., four volts, and the internal resistance that of oneohm divided by five + one ohm divided by five. Thus: E ^ 4 volts 4 o o = C. —^ =^ . r— = -T^ = 88 amperes. R-hr ■ 0-05-I-0*2+ 0*2 ohms 0-45 Six cells so connected are shown in Fig. 34. A battery of a given number of cells will send the strongestcurrent through a given external resistance when the cellsare connected (in series or in parallel, or partly in series Current-vStrength n and partly in parallel), so that the resistance of the batteryequals as nearly as possible the external resistance. (Thisbas been proved by differential calculus.) Thus, for gal-vanization small cells with a great internal resistance givealmost as strong a current as large cells with a low internalresistance ; while in cautery work, where the external
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Fig. -Six Cells in Compound Connection. resistance is very low, the internal resistance must also bemade low if satisfactory heating power is to be obtained.The most economical method will, however, be to chooseunder all circumstances cells, other things being equal,with as low an internal resistance as possible, and thereforeas large as may be convenient; for it is evident that what- 74 A Manual of Practical Medical Electricity ever energy is expended in overcoming an excessive in-ternal resistance is wasted, and that only that amount ofenergy which would be occupied in overcoming the lowestconvenient internal resistance can be considered to beusefully employed. High internal resistance is never anadvantage, though when the external resistance is alsohigh it may not be a material disadvantage.