480 ELECTRIC MEASUREMENTS [Cn. XIII 



the main part of the current goes through a special wire, known as 

 a shunt (fig. 254). 



In some ammeters the galvanometer, and the shunt are in the 

 same box (self-contained ammeters) , in others the shunt is outside 



(fig- 254). 



When an electric current flows through the ammeter, the 

 galvanometer needle is deflected, the amount of the deflection 

 measuring the amount of the current. With the ammeters used 

 in projection, the galvanometer has been calibrated so that the 

 needle points to the number of amperes of current flowing in a given 

 case (fig. 145). 



665. Connection of the ammeter with the projection circuit. 



If one is to use an ammeter in an electric circuit, the instrument is 

 connected with the line in series, that is along one wire. Further- 

 more, it is necessary to connect the positive pole of the ammeter 

 with the positive end of the wire, and the negative end with the 

 negative pole. In most cases when installing a projection outfit 

 the direction of the current flow is not known, and the proper 

 connection of the apparatus is found by trial (see 702 for the 

 direct current arc lamp). 



To install an ammeter cut one of the wires, and insert one cut end 

 in the positive, and the other cut end in the negative binding post 

 of the ammeter. Then the arc lamp and the rheostat are wired as 

 shown in fig. 270. 



Now close the switch and cause the arc lamp to burn. If the 

 ammeter is correctly connected, the needle will point to the number 

 of amperes of current flowing. If the connection is wrong, then 

 the needle will tend to move off the scale below the zero mark. In 

 case the connection is wrong, open the switch and reverse the posi- 

 tions of the wires in the binding posts of the ammeter. When the 

 current is turned on again the needle will be deflected until it 

 points to the number of amperes. 



By looking at fig. 273 it will be readily seen why one of the cut 

 ends of the same wire will be positive and why one will be negative. 

 That is, if the whole circuit is considered from the dynamo back to 

 the dynamo, it will be seen that starting from the positive pole of 



