Dynamo-electric Machines, 277 



descriptive, and partly devoted to the introduction of new 

 and ingenious arrangements. But, so far as can be judged 

 from the reports available, nothing has been added to the 

 theory, regarded from my point of view. 



I have not been able to experiment largely on the subject, 

 as it would necessitate the abandonment of other work ; and 

 there are other laboratories specially instituted for electrical 

 work, where this may more properly be done. But I have 

 made one successful experiment which is worth mentioning. 



My Gramme machine fails, as all main-circuit machines 

 fail, when it is attempted to drive a current through a high 

 external resistance. With any thing over 10 ohms the 

 machine fails. Now it was obvious that a comparatively 

 small shunt coil wound over the main coils would be well 

 excited in the case of high external resistance, and would 

 assist to start the action of the machine in these cases. 



The space available round the main coils was small ; but I 

 succeeded in winding nearly 2000 coils of a small wire on 

 pasteboard cylinders which fit over the main coils. The re- 

 sistance of the new coils is about 27 ohms. They are attached 

 to the armature-brushes, and form a shunt. They make a 

 great difference in the performance of the machine. 



Without joining up the main circuit at all, it is possible to 

 obtain an E.M.F. of 70 or 80 volts with the shunt circuit 

 alone. 



This is sufficient for all ordinary purposes, and is particu- 

 larly useful in charging accumulators ; for it obviates the 

 necessity of all the troublesome arrangements for starting 

 the charging which I have previously described to the 

 Society*, besides obviating the chance of reversal so long as 

 a reasonable amount of power is kept in action. 



It is no small experimental result that any Gramme machine 

 can have its capacity for work so greatly increased at the 

 small cost of time and material necessary to add to it shunt 

 coils of this description. 



The velocity and conditions of motion of a dynamo driven 

 by a prime mover depend on the equality of the power supplied 

 by the prime mover and the power absorbed by the machine. 

 We have therefore, first, to examine the nature of the re- 

 lation between the power supplied and the velocity, and, 

 secondly, the relation between the power absorbed and the 

 velocity. 



The relation between the power supplied by a prime mover 

 and the velocity is determined generally by a governor. 

 Whether this be the case or not, we can suppose the powers 

 * Phil. Mag. xiv. p. 250. 

 Phil Mag. S. 5. Vol. 15. No. 94. April 1883. X 



