. Archives of physical medicine and rehabilitation . )s a drop m volt-age along a line containing magnitude of this voltage drop de-pends both on the current and the re-sistance—the greater each of them, thegreater the drop. It follows, therefore,that if, at the main leads A and B i^n ^^^ ^^^ ^ ^ supply of sayi 10 volts, the voltage across C and D,the terminals of the primary of the higlitension transformer is less than 1 1 0 byan amount which depends on the re-sistance utilized in the rheostat and thecurrent strength. In the case of the gasbulb, this has a decided advantage ove
. Archives of physical medicine and rehabilitation . )s a drop m volt-age along a line containing magnitude of this voltage drop de-pends both on the current and the re-sistance—the greater each of them, thegreater the drop. It follows, therefore,that if, at the main leads A and B i^n ^^^ ^^^ ^ ^ supply of sayi 10 volts, the voltage across C and D,the terminals of the primary of the higlitension transformer is less than 1 1 0 byan amount which depends on the re-sistance utilized in the rheostat and thecurrent strength. In the case of the gasbulb, this has a decided advantage overthe auto-transformer control (See 3of this section). Suppose the resistanceof a gas bulb T (Fig. 25) suddenlylowers. (For reasons to be given laterthis may happen). This means that thehigh secondary voltage at once causes agreater current in the tube circuit, andconsequently automatically a greatercurrent flows through the primary (tosupply the necessary energy). Since,however, a rheostat is in the primarycircuit, this current increase means an. jr<.i/<? Fii; 3, increase m the voltage drop throughIhe rheostat, and therefore the voltageapplied to the primary of the trans-former lowers. As a result of this, thehigh tension voltage will lower alsonence the tube current will loweragain In this way a rheostat acts asa kind of safety valve preventing a sud-den increase of current beyond the ca-pacity of the outfit. This is not so inthe case of the auto-transformer. (2) Convolutions—In some unitsthe rheostat is supplemented by an ad-ditional control in the form of what arecalled convolutions. This simply meansthat the primary coil is wound in sec-tions, so that different numbers of turnsin the primary may be utilized. m Figure 26, it is evident thatby usmg termmals A and C, sections Iand ^ of the transformer would be util-i«d; by usirg A and D, sections I, 2and 3 would be utilized, and so ontiy such means the ratio between thenumber of turns of the primary a
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