494 Mr. 0. Heaviside on Resistance 



the resistance-operators of the four sides*, is in agreement 

 with the rule derived from (24) or (25) above, to make the 

 impulsive inductance of one combination vanish. Impulsive, 

 or " kick " balances, naturally require a galvanometer. Even 

 then, however, the method is sometimes unsatisfactory, when 

 the opposing influences which make up the impulse are not 

 sufficiently simultaneous, as has been pointed out by Lord 

 Eayleighf. 



There is also the striking method of cumulation of impulses 

 employed by Ayrton and Perry J, employing false resistance- 

 balances. It seems complex, and of rather difficult theory ; 

 but, just as a watch is a complex piece of mechanism, and is 

 yet thoroughly practical, so perhaps the secohmmeter may 

 have a brilliant career before it. 



Several interesting papers relating to the comparison of 

 inductances and permittances have appeared lately. It is 

 usually impulsive balances that are in question, probably 

 because it is not the observation of phenomena that is required, 

 but a direct, even if rough, measurement of the inductance or 

 permittance concerned, often under circumstances that do not 

 well admit of the use of the telephone. Only one of these 

 papers, however, contains anything really novel, scientifically, 

 viz. that of Mr. W. H. Preece, F.P.S.§, who concludes, from 

 his latest researches, that the " coefficient of self-induction " of 

 copper telegraph-circuits is nearly zero, the results he gives 

 being several hundred times smaller than the formula derived 

 from electromagnetic principles asserts it to be. Here is 

 work for the physicist. 



10. To equate the expressions for the electric and magnetic 

 energies of a combination is, I find, in simple cases, the easiest 

 and most direct way of furnishing the condition that the 

 impulsive inductance shall vanish. Thus, if there be but one 

 condenser and one coil, 8V 2 =LC 2 is the condition, S and L 

 being the permittance and the inductance respectively, V the 

 potential-difference of the condenser, and C the current in the 

 coil. The relation between V and C will be, of course, 

 dependent upon the resistances concerned || . But in complex 

 cases, and to obtain the value of the impulsive inductance 

 when it is not zero, equation (24) is most useful. 



* "On the Self-induction of Wires," Part VI., Phil. Mag. Feb. 1887. 



f ' Electrical Measurements,' p. 65. 



% Journ. Soc. Tel. Engineers and Electricians, 1887. 



§ B.A. Meeting, 1887 : " On the Coefficient of Self-induction of Iron 

 and Copper Wires." 



|| If the condenser shunts the coil, making V=RC, we get the case 

 brought before the S. T. E. & E. by Mr. Surnpner, with developments. 



