PRACTICAL OPERATION OF DYNAMOS. 183 



mica insulation between the commutator bars, and care must be 

 taken to distinguish these breaks from the partial or complete 

 breaks which may be found while the brushes make good con- 

 tact with the commutator bars. 



(V) Test for short-circuit. A short-circuited armature section 

 may perhaps be best located as follows : Pass a current from a 

 battery through the armature, using the regular brushes of the 

 machine. Then take a very low reading voltmeter (or a galva- 

 nometer), connect wire leads to it, and touch these wire leads 

 to one pair after another of adjacent commutator bars. A zero 

 or greatly reduced deflection of the voltmeter indicates a short- 

 circuited section. 



(d) Test for ground. A ground in the armature winding, 

 that is, a connection between the armature winding and the arma- 

 ture core may be located as follows : Connect a battery to the 

 dynamo brushes. Bring two leads from a very low reading volt- 

 meter (or a galvanometer) and connect one of these leads to the 

 armature shaft and the other to one after another of the commu- 

 tator bars. A deflection of the voltmeter indicates a ground, and 

 the ground is located near the commutator bar that gives the 

 least deflection. 



85 Rules for personal safety. Personal injury may result 

 from the passage of an electric current through the body, or from 

 severe burns due to a short circuit of the mains by a screw-driver 

 or other metal tool held in the hands. To avoid the first danger 

 the body must not be made a portion of an electric circuit , to 

 avoid the second danger never use a metal tool on live wires as, 

 for example, in the replacing of a burned out fuse. 



To make the body a portion of an electric circuit requires, of 

 course, two points of contact with the body. To avoid this it is 

 important to use one hand, only, in manipulating apparatus which 

 is connected with live wires, the other hand being put in the 

 pocket or held behind the back. It must not be forgotten, in 

 this connection, that the greatest risk lies in making a circuit 

 through the hand to the feet when one stands on a moist floor 



