26 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



effect of the electric current is also utilized in a variety of electric 

 furnaces. * 



Definition of the ohm. If H in equation (2) is expressed in 

 joules, f / in amperes, and t in seconds, then R is expressed 

 in terms of a unit which is called the ohm, that is, a wire has one 

 ohm of resistance when one joule of heat is generated in it in 

 one second by one ampere of current. The meaning of the 

 factor R in equation (2) may be made clear by solving this 

 equation for R, which gives R = Hj Pt. According to this 

 equation, the resistance of a wire in ohms is equal to the joules 

 of heat generated in it per ampere squared per second, or in 

 other words, an ohm is one joule-per-ampere-squared-per-second. 

 The abohm is defined in Art. 52. 



The international standard ohm. The resistance of a wire or 

 other portion of an electrical circuit can be measured with great 

 ease in terms of a known resistance, whereas a fundamental 

 measurement of resistance requires elaborate arrangements, and it 

 is very tedious if a moderate degree of accuracy is desired. 

 Therefore, for practical purposes, the ohm has been legally 

 defined | as the resistance at the temperature of melting ice of a 

 column of pure mercury 106.3 centimeters long, of uniform cross- 

 sectional area, and weighing 14.4521 grams. 



Measurement of resistance. A direct method for measuring 

 the resistance of a wire is to send a known current / through the 

 wire for a known length of time / and to determine the amount 

 of heat generated in the wire by means of a water calorimeter. 

 This direct method for measuring the resistance of a wire in 



* See Calcium Carbide Manufacture at Niagara, Electrochemical Industry, Vol. 

 I, page 22, and Carborundum Manufacture at Niagara, Electrochemical Industry, 

 Vol. I, page 50. See report of Canadian Commission on Electrothermic Processes 

 for the Smelting of Iron and Steel, by Eugene Haanel. 



f Ordinarily heat is expressed in terms of the calorie but it is desirable in the 

 present instance to express heat in joules, one joule of heat being an amount of heat 

 which is equivalent to one joule of work. One calorie is equal to 4.2 joules. One 

 joule of work is the amount of work done in one second by an agent which does work 

 at the rate of one watt. One watt is equal to 1/746 of a horse-power. 



J In accordance with the recommendations of the International Electrical Congress 

 which met at Chicago in 1893. 



