. Electricity : its medical and surgical applications, including radiotherapy and phototherapy . eadiness with which the con-ductor allows the passage of electricity. 1 From AndrS Marie Ampere, an eminent French scientist, who investigated the properties of theelectric current. In strictness it should be stated that the fundamental unit is really the ampere,which is defined by the magnetic effect of the current (see note, p. 67), and that the coulomb isderived from this as the quantity of charge transferred each second by one ampere. The German physicist, Georg Simon Ohm, ELECTRICAL RESISTANCE
. Electricity : its medical and surgical applications, including radiotherapy and phototherapy . eadiness with which the con-ductor allows the passage of electricity. 1 From AndrS Marie Ampere, an eminent French scientist, who investigated the properties of theelectric current. In strictness it should be stated that the fundamental unit is really the ampere,which is defined by the magnetic effect of the current (see note, p. 67), and that the coulomb isderived from this as the quantity of charge transferred each second by one ampere. The German physicist, Georg Simon Ohm, ELECTRICAL RESISTANCE 43 Unit of Resistance.—A conductor is said to have unit resistance whena unit E. M. F. will produce a unit current. In this case i, E, and R inthe formula above are each unity. In the practical system this unitis called the ohm, and is the resistance of a conductor in which thedifference of potential of one volt between its ends produces a currentof one ampere. As it is very desirable to have a concrete standard,careful experiments have determined that an ohm is represented by the Fig. 18. resistance of a cylindrical column of mercury cm. long and oftotal mass grams (so that the cross-section shall be 1 sq. mm.)when at a temperature of 0° C.^ For convenience of use, coils of wire having the same resistance (ormultiples of it) are used as working standards. Such a standard ohmis represented in section in Fig. 18. The heavy terminals are the elec-trodes where the E, M. F. is applied. Fig. 19 represents the construc-tion of a set of standard resistances, known as a resistance box. The 1 Legally in the United States the ohm is defined as above, the volt being defined in terms of theohm and ampere. 44 THE ELECTRIC CURRENT ends of the coils are connected to heavy metal blocks. Metal plugsinserted between the blocks short circuit the coils so that the currentpasses directly from block to block instead of through the coils. On theremoval of a plug, the current flows t
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