Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . ous electri-cal purposes is by inserting,or taking out of a circuit, re-sistance. It will be seen, from Ohms Law, C = |, that if the pressure (E) is constantthe current (C) can readilybe regulated by increasingor decreasing the value ofR ; that is, changing the re-sistance in circuit. An adjustable resistance or any apparatusfor changing the resistance without opening the circuit is calleda rheostat. The function of a rheostat is to absorb electricalenergy, and this energy, which app
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . ous electri-cal purposes is by inserting,or taking out of a circuit, re-sistance. It will be seen, from Ohms Law, C = |, that if the pressure (E) is constantthe current (C) can readilybe regulated by increasingor decreasing the value ofR ; that is, changing the re-sistance in circuit. An adjustable resistance or any apparatusfor changing the resistance without opening the circuit is calleda rheostat. The function of a rheostat is to absorb electricalenergy, and this energy, which appears as heat, is wasted in-stead of performing any useful work. A rheostat may beconstructed of coils of iron wire, iron plates or strips; car-bon, either pulverized in tubes or in the form of solid rodsor discs; German silver, platinoid, etc., wound on spools ;columns of liquids, as water and mercury, etc. The cross-sectional area of the material must be sufficient to carry thecurrent without excessive heating. In rheostats used for Fig. 123. -Commercial Type ofRheostat. CIRCUITS AND THEIR RESISTANCE. 149. Fig. 124.—Commercial Rheostat. Diagram of connections of theresistance coils. regulating the current in commercial electric circuits no great degree of accuracy of the resistance coils is required, as is the case in laboratory rheostats, ^j 165. Fig. 123 illustrates a commercial type of rheostat, inwhich the various coils are con-nected to brass buttons or con-tact segments. By moving ametallic connecting arm overthe segments the coils are thrownin or out of circuit, and the re-sistance thus readily adjusted,as shown in the diagram of con-nections, Fig. 124. In sometypes of rheostats the wire iswound around an iron frame-work which has been previouslydipped into a fireproof insulat-ing enamel. The advantage ofthis construction is that the heatfrom the wire is dissipated muchmore rapidly, so that a much smaller wire can be used to carry a given current. The size of such an
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