CH. XIII] 



POLARITY TESTS 



509 



position of the wires in the binding posts of the voltmeter. Turn 

 on the current and the voltmeter will register. It is well to mark 

 the insulation of the positive wire with red, or in some .other way. 



FIG. 272. VOLTMETER FOR TESTING POLARITY. 



G Dynamo for direct current. The positive pole is above and the negative 

 pole is below, as indicated by the arrows. 



Vm The terminals of the voltmeter are correctly connected across the line 

 (in multiple) or to both wires and the hand indicates the voltage on the dial. 

 If the terminals were wrongly connected the hand would not register. 



A Arc lamp. 



R Rheostat. 



The arrows indicate the direction of the current. 



The -}- and signs indicate that any point in the circuit nearer the positive 

 pole of the dynamo is positive to any point nearer the negative pole. 



FIG. 273. AMMETER FOR TESTING POLARITY. 



G Dynamo for direct current. The positive pole is above and the negative 

 pole below. 



Am Direct current ammeter. The terminals a +, b are connected along 

 one wire (in series). If the positive pole of the ammeter is connected to the 

 circuit next the positive pole of the dynamo, and the negative terminal in the 

 wire toward the negative pole of the dynamo, as here shown, the hand will 

 register when there is current flowing. If the connections are reversed the 

 hand will not register when the current is flowing. 



a+, b The positive and the negative terminals of the ammeter. 



A Arc lamp. 



c+ The positive carbon. 



c The negative carbon (the minus sign is put parallel with the carbon to 

 show the direction of the current). 



R Rheostat. 



The + and signs and the arrows are as with the voltmeter (fig. 272). 



