4 



Dr. S. P. Thompson on the Law of the 



armature-coils and to the intensity of the magnetic field. 

 Writing this as an equation, we have 



E = nM, 

 and, by Ohm's law, 



E = t'/R, 



which gives us for the current i, 

 or 



M~R 



Since M is itself a function of i, and not primarily of n or 

 R, it is clear that i is itself a function of n/R, and we may 

 write 



*<•?-£'■ 



This function Frolich set himself to investigate by pure 

 experiment without making any hypothesis whatever. He 

 determined the values of i 

 at various speeds and with 

 various resistances, and plot- 

 ted out the results as a curve, 

 values of i being taken as 

 ordiuates and values of n/R 

 as abscissae. This curve (see 

 figure) he termed the " cur- 

 rent-curve." It shows itself 

 to be very nearly a straight 

 line, which, however, does 

 not pass through the origin. 

 The portion which departs 

 from the line is difficult of 



observation because it is only obtained when either n is very 

 small or R very large, that is to say when the dynamo is 

 scarcely able to excite its magnets. Neglecting this unstable 

 state, and dealing only with that part of the curve which 

 relates to the machine as it is when in action, it is clear that 

 the relation between the two variables may be expressed as 



. n 

 6i=p — a, 



where b and a are constants ; giving us as the equation of the 

 series-dynamo : — 



i= l(n -<*} 



