Mr. R. H. M. Bosanquet on Practical Electricity. 247 



external resistances the total required would be larger ; and 

 this cannot be obtained. The load on the engine is heavy ; 

 full steam is required ; and the speed is moderate. 



Increase the 5 ohms to 6. leaving the steam as before. The 

 engine runs faster, and the required difference of potential is 

 kept up with about the same head of steam. 



Increase to 7 ohms. Similar effect. 



Increase to 10 ohms. The engine runs very fast; but the 

 required difference of potential is still easily maintained. Of 

 course here the total E.M.F. required is decidedly lessened. 



As the resistance increases beyond this point, the speed re- 

 quired to maintain a difference of potential of 80 volts becomes 

 too great for convenience, and by the time it reaches 20 or 

 30 ohms it becomes impossible. 



In view of this power of accommodation, the question as to 

 the precise resistance required to produce a given current at 

 a given speed becomes of little practical importance. 



The explanation of the power of accommodation is, roughly, 

 as follows. The total E.M.F. is to be supposed constant. In 

 the preceding experiments it is not quite so, being 100 volts, 

 about, with the 5 ohms, and 90 or so with the 10. But the 

 conditions of constancy can be realized; for with the 10 ohms 

 I kept up the difference of potential of the terminals to 90 volts 

 for some time, this corresponding to about 100 volts total 

 E.M.F. ; so that, in fact, this condition of constancy of total 

 electromotive force can be attained through a certain range 

 with a little attention. 



Then the current developed follows Ohm's law throughout 

 this limited range, with varying resistances; and the attraction 

 between the magnets and armature depends on the current in 

 each; so that a rough idea of the course of the values may be 

 formed by assuming the attraction proportional to the square 

 of the common current through both ; and this is the reaction 

 against the engine. Consequently as the resistance in circuit 

 increases the current diminishes (Ohm's law), and the statical 

 resistance to the engine diminishes, as we suppose for the 

 moment, in the square of the ratio of increase of resistance in 

 the circuit. Of course, the real values of the attraction depend 

 on the magnetizations of magnets and armature ; but the 

 above accounts in a general way for the increase of speed pro- 

 duced by introducing additional resistances into the circuit. 

 The increased speed is then utilized in obtaining the same 

 electromotive force from the diminished current, or rather 

 from the diminished magnetization. 



It will be readily seen how importantly this power of accom- 

 modation may be utilized in the practical treatment of such 



