34 PHYSIOLOGICAL PHYSICS, [Chap. iv. 



that Association in 1860 ; shortly it is called the B. A. 

 unit, or, after the discoverer of the laws of resistance, 

 the OHM. This resistance would be represented by 

 that offered by a column of pure mercury 104 -SI cen- 

 timetres long, 1 square millimetre in section, at a 

 temperature of Centigrade. For practical use 

 coils are made of wire of such leno-th and thickness 



o 



that they offer precisely this amount of resistance. 

 Thus a coil is made of wire formed of an alloy of two 

 parts of silver and one of platinum. The wire is 

 from one to two metres long, and from '5 to '8 

 millimetres in diameter. The wires are soldered to 

 thick copper electrodes, and properly insulated by silk 

 and paraffin. At a given temperature, which requires 

 to be determined for the particular coil, it will offer the 

 standard resistance, and will be, therefore, equal to 

 one ohm. The exact temperature at which it gives 

 this resistance should be marked on the coil.* Simi- 

 larly boxes may be made containing different coils of 

 wire of varying lengths, so as to offer varying resis- 

 tances. The amount of resistance each coil offers is 

 marked on it in ohms ; and these can be made use 

 of to interpose any given resistance in the path of 

 a current. (See chap, xiii.) 



CHAPTER IV. 



INDUCTION AND INDUCTION COILS. 



IN 1831 Faraday discovered that a current flowing 

 along a closed circuit was able to produce a current in 

 a neighbouring coil of wire simply through influence. 



* Refer to Clerk-Maxwell on " Electricity and Magnetism," 

 vol. i. p. 390. 



