Mr. It. H. M. Bosanquet on Practical Electricity. 245 



sions a farther resistance of 5000 ohms is interpolated in the 

 galvanometer circuit. Brass dial 3 inches in diameter divided 

 to degrees. Short steel needle with long pin-points. 50° 

 on the galvanometer, with the 5000 ohms external resistance, 

 corresponds very nearly to 50 volts. Other readings by the 

 tangent-law. 



Two Quantity Galvanometers. — These consist each of a ring 

 of gun-metal, "11 m. radius; the conductors leading to them 

 are copper, and of tube-and-core form; brass dials, and steel 

 needles with long pin-points as before. 



When dealing with large currents I placed a steel magnet 

 under one of these, so as to convert it into a high-quantity 

 instrument. Its constant was ascertained by sending the 

 same current through both instruments, the one without the 

 magnet reading as a tangent galvanometer in absolute mea- 

 sure. 



Measures. 



Although numerous measures of dynamo machines have 

 been published, the laws of any given machine cannot as 

 yet be predicted; and a few measures are given, which are 

 sufficient to illustrate the general course of the performance 

 of this machine under different circumstances. 



First, we will consider the cases where the governor was 

 used. This may be taken to give 840 revolutions in all cases, 

 except where, the resistance being 3 ohms or less, the leverage 

 against the engine was such as to reduce the speed materially. 



Resistance in parallel circuit 



with lamp. 



ohrns. 



Tension between 





terminals. 



Quantity 



volts. 



ampere3, 



24-5 



3-8 



31-4 



5*5 



37-9 



7-6 



41-0 



10-5 



5 

 4 



With lower resistance the speed cannot be maintained with- 

 out taking the governor off. In considering the total electro- 

 motive force developed, of course the whole resistance of the 

 circuit must be considered. The resistance of machine and 

 connexions may be taken at 1/5. Then, multiplying the 

 current by the whole resistance, we should have the total 

 electromotive force. 



The work of Meyer and Auerbach leads for a given speed 

 to equations of the form # = aE— b, where 6 is an angle 

 whose tangent is the measure of the current, and E the total 

 electromotive force. I used the expression at first as it stands; 



