246 Mr. E. H. M. Bosanquet on Practical Electricity. 



but now I think it better to eliminate E as follows: — Putting 



E = CR, the above form gives CR = orR= — rr, 



' ° a a \j 



where R is the resistance of the circuit, C the current, and 

 the angle of a tangent-galvanometer by which the current is 

 measured. 



To apply this to the above observations, we find the follow- 

 ing values of the total resistance, current, and 0:- 









0-20. 



E. 



C. 



6. 





7-9 



3-8 



50° 



7-9 



7-0 



5-5 



60 



7-3 



&2 



7-6 



67i 



G-2 



5-3 



10-5 



73 



5-1 



(When calculating the resultant resistance of the parallel 

 circuit, we notice that it should be the same as the quotient of 

 the observed tension by the current. It does not differ in 

 any case by more than one or two tenths of an ohm. This 

 is sufficient, considering that no special accuracy was aimed 

 at. Further, it must be remembered that the wires of the 

 resistance are nearly or quite red-hot under these circum- 

 stances, so that some discrepancy is to be expected.) 



Calculating R from the formula 



6-20 

 R=- 7y -, 



we obtain the numbers in the last column. 



This result is tolerably satisfactory; but it is of little prac- 

 tical use where the motor employed is a steam-engine capable 

 of considerable variations of speed. With a gas-engine, or 

 other motor of very constant speed, the above method of find- 

 ing the approximate resistance for a given current would 

 probably be useful. 



But with my engine, when the governor is not used, the 

 speed adapts itself to the work to be performed; so that, with- 

 in considerable limits, the only things to be considered are the 

 amount of steam turned on and the work to be done. To 

 illustrate this I will quote a series of experiments, in which 

 the object was to maintain the tension necessary for a 

 " British " incandescent lamp, which is about 80 volts, with 

 different resistances in parallel circuit. 



The least resistance with which this tension can be main- 

 tained is 5 ohms. It is easily seen that the resulting external 

 resistance here is nearly four times that of machine and con- 

 nexions; so that the total E.M.F. would require to be 100 

 volts, roughly, to get 80 between the terminals. For smaller 



