a 
forces would be produced by the discharge of a condenserin this way, but not 
by leakage. In this case the theory differs from Maxwell’s, as according 
to that theory the alteration in the electromotive force would produce 
magnetic forces in either case. 
In Professor Poynting’s second paper, which we have already men- 
tioned, the fundamental principles of electrodynamics are described as 
the results of the motion of the tubes of electromotive and magnetic 
force. Maxwell develops electrodynamics from the principles :— 
Ist. That the total electromotive force round any closed curve is 
equal to the rate of decrease of the total magnetic induction through the 
curve. 
2nd. The line integral of the magnetic force round any closed curve 
is equal to 47 times the current through the curve. 
Professor Poynting restates these principles in the following way :— 
1. ‘Whenever electromotive force is produced by change in the mag- 
netic field, or by motion of matter through the field, the E.M.F. per 
unit length is equal to the number of tubes of magnetic induction 
cutting or cut by the nnit length per second, the H.M.F tending to 
produce induction in the direction in which a right-handed screw would 
move if turned round from the direction of motion relatively to the tubes 
towards the direction of the magnetic induction.’ 
The second principle he states in the following way :— 
_ * Whenever magnetomotive force is produced by change in the electric 
field, or by motion of matter through the field, the magnetomotive force 
per unit length is equal to 47 times the number of tubes of electric 
induction cutting or cut by unit length per second, the magnetomotive 
force tending to produce induction in the direction in which a right- 
handed screw would move if turned round from the direction of the 
electric induction towards the direction of motion of the unit length 
relatively to the tubes of induction.’ 
_ By magnetomotive force is meant the line integral of the magneto- 
motive force round a tube of induction. This statement includes the 
more special one that the line integral of the magnetic force round any 
closed curve is equal to 47 times the number of tubes passing in or out 
through the curve per second. 
The development of these principles leads to equations which are 
practically the same as those obtained by Maxwell, the chief difference 
being that the quantity corresponding to Maxwell’s J is no longer 
arbitrary or rather redundant. 
Professor Poynting also introduces into his equations the time 
integrals of the components of the magnetic force as fundamental quan- 
tities, and regards the components of the magnetic as the differential 
coefficients of these quantities with regard to the time. This method of 
representing magnetic force was also used by Professor Fitzgerald in his 
paper on the Electromagnetic Theory of the Reflection and Refraction 
of Light.! It has the advantage of calling attention to the dynamic 
character of magnetic phenomena. In Professor Poynting’s paper some 
of the applications of his method of regarding electrical phenomena are 
worked out with great detail for some of the simpler cases. 
ON ELECTRICAL THEORIES. 153 
1 Phil. Trans., 1880, part ii. 
