CH. XIII] 



ELECTRIC MEASUREMENTS 



479 



main current continues to flow in the electric circuit between the 

 two points exactly as though the voltmeter were not in use. 



The voltmeter should not be connected in series with the line as 

 with the ammeter ( 665). While no particular harm might be 

 done, the high resistance of the voltmeter would allow only a small 

 current to flow in the line and if one were using an arc lamp it 

 would go out from the insufficient current. 



If one does not know the direction of the flow in the circuit to be 

 tested, the voltmeter can be correctly connected by trial as follows : 

 Connect the positive binding post of the voltmeter by a wire to one 

 of the points, and the negative binding post by a wire to the other 

 point. Turn on the current, and if the connection is right the 

 needle of the instrument will point to the voltage ; if the connection 

 is wrong the needle will tend to be deflected off the scale below the 

 zero point. If it is wrong, turn off the current and reverse the 

 position of the wires in the binding posts. 



664. Ammeter. This is 

 an instrument to show the 

 amount of current flowing 

 through a given circuit at a 

 given instant. It consists of a 

 galvanometer of the particular 

 type adapted to the current 

 used, that is, direct or alter- 

 nating current. It is also 

 adapted to the amount of cur- 

 rent to be measured, that is 

 small currents and large cur- 

 rents, say from o to 10, o to 25, 

 o to 50, o to 100, etc. 



The galvanometer part of the 

 ammeter is a delicate instru- 

 ment so that the whole current 

 used in projection is not sent 

 through it, but a definite frac- 

 tional part goes through it and 



FIG. 254. CONNECTING AN AMMETER 



IN THE LINE TO MEASURE THE 



CURRENT FLOWING. 



a Ammeter, A, with external shunt, 



S. 



b Ammeter with self-contained 

 shunt. The shunt in this type is in- 

 side of the instrument case. 



Note, the ammeter is connected 

 along one wire so that the entire cur- 

 rent flows through the instrument and 

 its shunt. 



