59^ ELECTRICAL UNITS. 



Accordingly the Committee of the British Association, founded in 

 1 86 1 for the purpose of establishing a rational system of electrical 

 measurements, was led to choose units more suited for practical 

 needs, and to give special names to these units so as to facilitate 

 their adoption. This system has been ratified in the following 

 form by the Congress at Paris : 



The practical unit of resistance is equal to io 9 C.G.S. absolute 

 units, and acquires the name of Ohm* 



The Volt is the practical unit of electromotive force ; it is equal 

 to io 8 C.G.S. units. f 



The Ampere is the current produced by the electromotive force 

 I of a volt in a circuit having the resistance of an ohm ; it is equal 

 to io- 1 C.G.S. 



The Coulomb is the quantity of electricity which, in a second, 

 traverses the section of a conductor which is conveying the current 

 of an ampere; the coulomb is equal to ro" 1 C.G.S. 



The Farad is the capacity of a condenser whose armatures 

 acquire a difference of potential of one volt when the charge is a 

 coulomb; the farad is io~ 9 C.G.S. units. 



In certain applications it is useful to express different magni- 

 tudes by means of units which are a million times as small or as 

 great as the corresponding practical unit. These new units are 

 called by the same name with the prefix mega or micro, according 

 as they are multiplied or divided by a million, 



Thus the megohm is equal to io 6 ohms ; the submultiple called 

 the microhm is io~ 6 ohms. In like manner, for capacity, the 

 millionth of the farad, or the microfarad, is equal to io~ 6 farads 

 or io- 15 C.G.S. units. 



* The Committee of the British Association made numerous experiments in 

 order to determine the value of the Ohm and to construct material standards 

 presenting the same resistance. The first investigations appeared to show that 

 the Ohm is represented with a close degree of approximation by the resistance 

 of a column of mercury at 0, a square millimetre in cross section, and 104 centi- 

 metres in length ; but it appears that certain errors were made in the calculation, 

 and that this length should be increased by about one per cent. that is, raised 

 to 105 centimetres. Recent investigations lead to the same result, but the 

 question does not seem to have been definitely solved. As it is not certain, on 

 the other hand, that solid metals do retain their electrical properties without 

 change, the Congress decided that the Ohm should be represented by a column of 

 mercury at zero, having a cross section of one square millimetre, and that an 

 international commission should settle by fresh experiments the exact length 

 of this unit. 



f The electromotive force of a Daniell with sulphuric acid is about 1.08 volts. 



