. A text-book of radiology . evice we connect one wire to one end of theresistance, and the other is usually connected to a metalslider that may be moved along from end to end, touchingall the wires in turn. When it is at*the same end as thewire already connected, the voltage between them is, ofcourse, nil, but as we move the slider towards the other 8 TEXT-BOOK OF RADIOLOGY. end the difference is one volt for every turn of wireseparating them, and it remains the same for any givenposition, whether current is being used in this secondcircuit or not. The principal use for a shunt resistanceis t
. A text-book of radiology . evice we connect one wire to one end of theresistance, and the other is usually connected to a metalslider that may be moved along from end to end, touchingall the wires in turn. When it is at*the same end as thewire already connected, the voltage between them is, ofcourse, nil, but as we move the slider towards the other 8 TEXT-BOOK OF RADIOLOGY. end the difference is one volt for every turn of wireseparating them, and it remains the same for any givenposition, whether current is being used in this secondcircuit or not. The principal use for a shunt resistanceis to make electrical applications to patients, and as thecurrents required are very weak, they can be made verysmall and compact. For this purpose they are generallywound with fine wire and have about ten turns for eachvolt. This enables us to vary the potential by tenths of avolt, and the resulting flow of current to the patient iscorrespondingly delicate. A diagram of the usualarrangement is shown here. (Fig. 1.) The lamp and. Fig. 1. Plan of Shunt Resistance. fuse are for safety, and the switch is to disconnect it fromthe main when no longer required. While shunt resistances are not much used in radiology,they have many advantages, especially when workingwith mercury interrupters from high voltage mains, 200to 250 volts. They have necessarily to be made large andthey waste a lot of current, but considering the low priceit is supplied at for such work as this, it does not amountto very much. The circumstances under which theywould be advisable will be indicated as they arise. Electro-Magnetic Induction.—If we take a wirethrough which a strong current is passing and dip it into MAGNETIC INDUCTION. 9 iron filings, some of the latter will be found sticking to itand will not readily fall off so long as the current con-tinues to flow. If the current is cut off, the filings fallaway at once and will not again attach themselves to thewire until the current is restored. The reason for
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