200 ELEMENTS OF ELECTRICAL ENGINEERING. 



(see Art. 75), it is usual to make a test of a completed station, 

 including boilers and engines to determine the economy of the 

 station as a whole to make sure that a certain station economy, if 

 guaranteed, is realized. 



89. Ammeters. An ammeter is a galvanometer with a pointer 

 which plays over a divided scale and indicates, directly, the value 

 of the current flowing through the instrument In many cases a 

 small but definite fractional part, only, of the current flows through 

 the ammeter, the remainder flowing through a low resistance 

 shunt. The numbers on the ammeter scale may be made to give 

 the value of the total current including that which flows through 

 the shunt. If desired, the instrument may be provided with 

 several interchangeable shunts of different resistances, so that the 

 instrument may be used to measure large or small currents at 

 will. In such a case the scale readings must be multiplied by a 

 factor to give the total current, and this factor has a large value 

 for a low resistance shunt and a small value for a high resistance 

 shunt. 



There are five distinct types of ammeters, namely : 



(a) The tangent-galvanometer type in which the pointer is at- 

 tached to a small permanent magnet which is deflected by the 

 current to be indicated. This type is now seldom used. 



(b) The D'Arsonval-galvanometer type in which the current 

 to be indicated (or a definite fractional part of it) flows through a 

 small suspended or pivoted coil which is deflected by a perman- 

 ent steel magnet. The pointer is attached to the pivoted coil. 

 This type is much used for precision ammeters for direct currents. 

 The Weston direct-current ammeters are of this type. 



(c) The electrodynamometer type, in which the current to be 

 indicated (or a definite fractional part of it) flows through a 

 stationary coil and a movable (pivoted) coil connected in series. 

 The force action between the two coils deflects the pivoted coil 

 and causes an attached pointer to play over a divided scale. This 

 type is much used for precision ammeters for alternating current ; 

 it is suitable also for direct current, but it has the disadvantage 



