Governing of Electromotors. 65 



is equal to A, equations (1), (3), and (4) lead to 



n\p±qA±t(G-A)\=(G-A)z-A(a+s). . (6) 



Equations 5 and 6 have then to be satisfied for all values of 

 A for constant values of n and C ; hence in the case of a short 

 shunt, 



±n(-tA)=-A(z + a), ..... (7) 



and in the case of a long shunt 



±n(q-t)A=-A fc+a + s). .... (8) 



But t, q, z, a, and s are all positive quantities, and since t 

 is always larger than q, that is the number of turns in the 

 shunt-coil is always larger than the number of turns in the 

 series-coil, it follows that to satisfy either equation (7) for the 

 short shunt or equation (8) for the long shunt, the positive 

 value of t in the original equation (1) can in either case alone 

 be employed. In other words, whatever be the arrangement 

 of the series-coil, the shunt-coil must be a magnetizing and 

 not a demagnetizing one. 



This result we pointed out in our original paper on 

 " Electromotors and their Government," in vol. xii. of the 

 1 Journal of the Society of Telegraph Engineers and Elec- 

 tricians,' since on page 314 we stated that the solution for 

 constant current was a shunt-motor with a series demagne- 

 tizing-coil; and it would be unnecessary to refer to this point 

 again were it not that in one of the standard English treatises 

 in which our method of governing motors is described in 

 detail, the only solution for constant current that we are 

 credited with is the impossible one of a series-motor with 

 shunt demagnetizing-coil, the author not having observed that 

 such a combination, whether used as a short or as a long 

 shunt, could only make a motor run at a uniform speed when 

 the ^peed was negative. Now, what does a negative value of 

 n mean? It means that the armature must run the other 

 way, or, in other words, that what was supposed to be a 

 series-motor must in. reality be the armature combined with 

 a series demagnetizing-coil, that is must act as a series brake- 

 dynamo, and the supposed shunt demagnetizing-coil must, in 

 conjunction with the armature, apt as a shunt-motor; or, in 

 other words, the solution is as wo originally stated it — a 

 >hunt-motor with series demagnetizing-coil. 



In another important English work our method is 

 given correctly, but it is stated that the method would not 

 work, because when a greater load was put on the motor the 

 armature would go more slowly, the back E.M.F. would 

 therefore be diminished, and less current would pass round 

 Phil. Mag. S. 5. Vol. 26. No. 158. July 1888, F 



