1889-90.] Sir William Thomson on Ohmic Resistance. 159 
“ effect upon the exposed parts of my hand, but nothing was scorched 
“ except the skin at the points of grasp. 
“ The dynamo was of Crompton’s pattern, with compound wind- 
“ ing. The speed was about 1300 revolutions per minute. The 
“ dynamo was not employed in charging batteries, but in the direct 
“ lighting of incandescent lamps. The duty of the dynamo at the 
“ time would be about 85 amperes and the potential 103 volts. No 
“ check in the dynamo was perceived, nor was it likely to be observ- 
“ able, seeing that the momentum of the revolving parts would be 
“ enormously powerful to overcome any momentary increase of re- 
“ sistance. There being two dynamos on one axis, both in motion, 
“ though only one was doing work, besides the turbine- wheel and 
“ the impinging jets, there would be a collective momentum of great 
“ energy for a momentary effort.” 
The requisites for working out fully the theory of transient or 
periodic currents in conductors of any form, are included in 
Maxwell’s fundamental equations of electro-magnetic induction, 
and are given explicitly for straight cylindric conductors of non- 
magnetic material in §§ 685-689 of his great work. Lord Ray- 
leigh in his paper “ On the Self-Induction and Resistance of Straight 
Conductors” {Phil. Mag ., May 1886) gives explicitly the proper 
formulas for transient or periodic currents, in straight cylindric 
rods of iron, on the supposition of constant magnetic susceptibility. 
The details of this highly interesting and important branch of the 
subject have also been investigated by Heaviside in a very compre- 
hensive manner. The tendency of periodically alternating currents, 
to be condensed in the outer part of a cylindric conductor, while the 
current may be exceedingly feeble or quite insensible in the central 
parts, was discussed an^ explained by Lord Rayleigh in p. 388 of 
his article already referred to, and its aggravation in an iron con- 
ductor specially pointed out. The same considerations show that a 
transient current resulting from the application, for a very short 
time, of the electro-motive force of a voltaic battery, or of electro 
magnetic induction acting not directly on * the cylindric conductor 
* This caution is introduced to avoid leading any reader into an error 
into which I myself fell in the text, and corrected in footnotes, of an article 
in Philosophical Magazine for March 1890, “On the Time-Integral of a 
Transient Electro- Magnetically Induced Current.” 
