128 



Trans. Acad. Sci. of St. Louis. 



lib. 



The Dynamo as a Speed Indicator. 



In a great deal of experimental work it is essential that we 

 determine the instantaneous values of the speed of the appa- 

 ratus. This can be very easily done by using a dynamo 

 electric machine attached to the revolving shaft. 



22.0 



21.0 



n 



20.0 



19.0 



18.0 



2.60 



3.00 



Fig. 14 



3.50 



4.00 



Series Dynamo Test. Relation between the Currentand Speed 

 when the Resistance was Constant. 



(1) If it is possible to use a separately excited dynamo 

 with an electrostatic voltmeter connected to its brushes, then 

 the speed is directly proportional to the potential shown by 

 the voltmeter. Care must, of course, be taken to keep the 

 exciting current constant. 



Equation (1) p. Ill gives this relation. 



E = 4 AHnj = n x multiplied by a constant. 



(2) This can also be more simply obtained by using a self- 

 excited series dynamo and either an ammeter in the circuit or 

 a voltmeter across the brushes. 



A very convenient machine for this purpose will be a 



