Relations between Electrical Measurements. 525 



The electromagnetic unit of quantity of electricity is equal to 

 the electrostatic unit multiplied by a certain velocity, depending 

 on the elasticity of the magnetic medium, and proportional or 

 probably equal to the velocity of propagation of vibrations in it. 

 Hence the dimensions of 



Electrostatic quantity .... ^=LT 



Electrostatic current c=L 



Electrostatic electromotive force . e= -^ 

 Resistance r= ^ 



As we have no knowledge of the density, elasticity, &c, of the 

 magnetic medium, we assume it as having a standard state in 

 common air ; and supposing all measurements to be made in air, 

 the original table of dimensions is sufficient for expressing mea- 

 surements made according to one system in terms of any other 

 system. 



57. Magnitude of Units and Nomenclature. — In connexion 

 with the system of measurement explained in this treatise, two 

 points hitherto unmentioned deserve attention — first, the abso- 

 lute magnitude of the units, and secondly the nomenclature. 



The absolute magnitude is in most cases an inconvenient one, 

 leading to the use either of exceedingly small or exceedingly 

 large numbers. Thus the units of electromagnetic resistance 

 and electromotive force and quantity, and of electrostatic cur- 

 rents, are inconveniently small ; the unit of electrostatic resist- 

 ance is inconveniently large. Decimal multiples and submulti- 

 ples of these units will therefore probably have to be adopted in 

 practice. The choice of these multiples and submultiples forms 

 part of the business of the Committee. 



The nomenclature hitherto adopted is extremely defective. In 

 referring to each measurement, we have to say that the number 

 expresses the value in electrostatic or electromagnetic absolute 

 units : if a multiple is to be used, this multiple will also have to 

 be named ; and further, the standard units of length, mass, and 

 time have to be referred to, inasmuch as some writers use the 

 pound and some the grain, some the metre and some the milli- 

 metre, as fundamental units. This cumbrous diction, and the 

 risk of error imported by it, would be avoided if each unit 

 received a short distinctive name in the manner proposed by Sir 

 Charles Bright and Mr. Latimer Clark, in a paper read before 

 the British Association at Manchester, 1861. 



