. X-ray manual : Army . put in new mercury. The latteris expensive in those using large quantities but not inothers. A coil in which a current is changing always developsan active opposition to the alteration of current. On anattempt to increase the current, the coil acts as an op-posing generator, and when current falls the generatoraction reverses. This action is due to self opposing voltage when we change current at therate of 1 ampere per second is an important factor inbehavior of the coil, and is named the coefficient of selfinduction. On account of self induction, no
. X-ray manual : Army . put in new mercury. The latteris expensive in those using large quantities but not inothers. A coil in which a current is changing always developsan active opposition to the alteration of current. On anattempt to increase the current, the coil acts as an op-posing generator, and when current falls the generatoraction reverses. This action is due to self opposing voltage when we change current at therate of 1 ampere per second is an important factor inbehavior of the coil, and is named the coefficient of selfinduction. On account of self induction, no really instantaneouschange of current can take place, and the response tovariable voltage will depend on this feature of the coiland on the rate at which we attempt to make currentchanges. Each coil is an individual in this respect andone should find by trial the conditions under which itoperates best for each purpose, and then adhere to theseconditions. A little time spent in this way will savemuch time and annoyance Fig. 24. Oscillogram—Induction coil currents with Wehnelt X-RAY PHYSICS 55 Tubes for Use with Coils. The current wave through a coil is quite different fromthat from a transformer. The current consists of a seriesof short rushes with considerable time between each im-pulse. Fig. 23 shows the variations of current with timeon an induction coil with a good mercury 24, two curves with a Wehnelt break. Note thelarge amount of inverse in the latter. In order that thetube current may not lower the voltage below the re-quired point it is essential to have gas tubes at relativelyhigh vacuum, or hard. Thus we must have small tubecurrents. Readings. Milliampere and spark gap readings are far less re-liable guides for radiography when using coils than ontransformers. The gap shows peak voltage which maybe high but transient, the ordinary milliammeter indi-cates the difference between direct and inverse. Sothat one may get 0 reading a
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