File:Practical electro-therapeutics and X-ray therapy - with chapters on phototherapy, X-ray in eye surgery, X-ray in dentistry, and medico-legal aspect of the X-ray (1912) (14570398340).jpg

Identifier: practicalelectro00mart (find matches)
Title: Practical electro-therapeutics and X-ray therapy : with chapters on phototherapy, X-ray in eye surgery, X-ray in dentistry, and medico-legal aspect of the X-ray
Year: 1912 (1910s)
Authors: Martin, James Madison, 1866-1947
Subjects: Electrotherapeutics X-rays Diagnosis, Radioscopic Eye Electric Stimulation Therapy X-Ray Therapy Ophthalmologic Surgical Procedures
Publisher: St. Louis : C.V. Mosby
Contributing Library: Francis A. Countway Library of Medicine
Digitizing Sponsor: Open Knowledge Commons and Harvard Medical School

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The wire may be wound in astraight coil, or the coil may be bent into a circle or made in theshape of a horseshoe. Fig. 38 is a diagrammatic drawing of the 72 PRACTICAL ELECTRO-THERAPEUTICS AND X-RAY THERAPY simplest form of series rheostat. We will assume that the batteryB has an EMF of 60 volts. The rheostat B has a resistance of3,000 ohms, a connects the negative terminal of the battery to theresistance coil B. The contact shoe H is made to slide on the rod 8so that it can be brought in contact with any single wire on B. Asthe shoe now stands, no current can get into the resistance coil B.It is allowed to pass on to the external circuit and through coil P,which is made to represent the patient, and on through conductor5 and back to battery B. When the shoe H is moved to the otherend of the coil B, the whole resistance of the coil B is in the cir-cuit and the patient P will get the least possible part of the current.From this arrangement it will be seen that any strength of current
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Fig. 39.—Shunt rheostat. may be carried to the patient by moving the shoe H on the coil Bso as to bring as few or many coils of the rheostat into the circuitas the case may require. Should the resistance of P be the sameas the rheostat B, 3,000 ohms, it would not be necessary to movethe shoe H from its first position, but, when the patients resistancefalls, the rheostat will not only be a great convenience, but an ab-solute necessity. Where batteries are used, the series form of rheostat will answerall purposes, but when the current is taken from the commercial cir-cuit, to guard against a possible short on the main line, a shuntedrheostat (Fig. 39) should always be used. This form of rheostatdiffers from the series variety in that the resistance coil B is in CONTROLLING AND MEASURING APPARATUS 73 metallic contact with a and b. When the patient P is connectedin such a circuit, should the mains become shorted with a high-pressure current, the bulk of the charge would be short-circuit

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