66 Profs. Ayr ton and Perry on the 



the shunt-coil of the field-magnet. Hence the field-magnet 

 would be weakened, which would cause the motor to go more 

 slowly, and so on until it stopped and possibly reversed. This 

 objection is a curious one, because exactly the same apparent 

 difficulty might have been stated by the author of this treatise 

 when he was dealing with our method of governing motors 

 to run at a constant P.D. a few pages earlier. Here, how- 

 ever, he sees the explanation of the difficulty, and he points 

 out quite correctly that, although it might at first sight appear 

 anomalous to say that, in order to keep a motor running at a 

 fixed speed, it is necessary to weaken the field of the motor 

 when the load on it is increased, the explanation is found in 

 the fact that the power depends not merely on the strength of 

 the field but on the current passing through the armature, and 

 that this latter is much increased when the field is weakened. 

 Hence, on the whole, it is quite right to arrange matters so 

 that the field is weakened when the load is increased. We 

 say it is curious then, that while the author has explained the 

 apparent difficulty so clearly when he was dealing with our 

 method for governing motors when the supply condition was 

 a constant P.D., he does not see that exactly the same explana- 

 tion clears away the difficulty which he has himself raised 

 regarding our method for governing motors to run when 

 supplied with constant current. 



What, however, we particularly wish to point out in this 

 short note is that when the condition of supply is constant 

 current it is not necessary to have a series demagnetizing-coil 

 at all, and consequently the motor can be made to run at 

 constant speed independently of the load, and in addition to 

 work with high efficiency. 



We will first see whether such a solution is possible when 

 the condition of supply is constant P.D. Let the motor be a 

 simple shunt-motor; then, if as before E be the back E.M.F., n 

 the speed, p a term depending on the permanent magnetism, 

 and Z the current round the shunt, 



E = n(p + tZ); (9) 



also E = Zs — Aa, 



if z be the resistance of the shunt-coil, A the current in the 

 armature, and a the resistance of the armature ; also 



where V is the constant P.D. maintained at the terminal, of 

 the motor ; / V\ 



••• n(p + tj)=Y-Aa. . . . (10) 



