524 Prof. Maxwell and Mr. F. Jenkin on the Elementary 



Absolute mechanical equivalent of the unit of heat =4157*25 

 =423*542 metre- grammes at Manchester. 



Thermal equivalent of an absolute unit of work = 0*00024054. 



Thermal equivalent of a metr-grm . at Manchester = 0*00236154. 



Electrochemical equivalent of water =0*0092, nearly. 



56. Note to the Table of Dimensions, by Professor Clerk Max- 

 well.— All the measurements of which we have hitherto treated 

 are supposed to be made in the same medium — ordinary air; 

 but Faraday has shown that other media have different proper- 

 ties. Paramagnetic bodies, such as oxygen and salts of iron, 

 when placed in media less paramagnetic than themselves, behave 

 as paramagnetic bodies ; but when placed in media more para- 

 magnetic than themselves, they behave as diamagnetic bodies. 



Hence magnetic phenomena are influenced by the nature of 

 the medium in which the bodies are placed, and the system of 

 units and of measurements which we adopt depends on the 

 nature of the medium in which our experiments are made. If 

 we made our experiments in highly condensed oxygen, magnets 

 would attract each other less, and currents would attract each 

 other more, than they do in common air; and the reverse would 

 be the case if we worked in a sea of melted bismuth. 



Now if we take into account the "coefficient of magnetic 

 induction " of the medium in which we work, and instead of 

 assuming that of common air to be unity, assume it proportional 

 to the density of that part of the medium to which the mag- 

 netic action is due, we shall have the repulsion of two poles 



Tnm 

 = — ^-, where mm' are the two poles, fi the density of the 



magnetic medium, and r the distance. Now a density is a mass, 

 Mj, divided by L 3 , the unit of volume. Hence the dimensions of 



/MM 



m are\/ Ta 1 ; or if we can measure the density of the mag- 

 netic medium in the same unit of mass as that employed for 



M 

 other purposes, the dimensions of m will be simply -p^-; those 



of H will then be ^, or a velocity. 



If we suppose the density of the magnetic medium to be taken 

 account of in the electromagnetic units, their dimensions become 

 Quantity of electricity . Q = L 2 , or equivalent to an area. 



L 2 



Strength of current . . C = 7^- 



M 



Electromotive force . . E = ^ 



M 



Resistance of conductor R= ^m 



