PRACTICAL OPERATION OF DYNAMOS. 2OI 



that the deflecting forces are small, so that the instrument must 

 be very finely constructed. Another disadvantage of this type 

 when used for direct currents is that the direction of deflection is 

 not reversed by a reversal of the current. 



(d) The hot wire type in which the current to be indicated 

 flows through a fine wire, which by its rise of temperature and 

 consequent expansion, actuates a pointer which plays over a 

 divided scale. This type of instrument, although very cheap in 

 construction, has not yet come into very general use on account 

 of its inconstancy. 



(e) The plunger type. This type includes all of the great 

 variety of ammeters in which a piece of soft iron is magnetized 

 and deflected by a coil of wire through which the current to be 

 indicated, flows. The simplest form of the plunger type of 

 ammeter is that from which it takes its name, namely a coil of 

 wire and a soft iron plunger which is drawn into the coil by the 

 current, in opposition to the pull of a spring or to the pull of 

 gravity. In the most approved form of plunger ammeter a soft 

 iron vane is attached to a pivot which is controlled by hair 

 springs, and the instrument is so designed as to magnetize the 

 soft iron vane strongly, even when the current to be indicated is 

 a small fraction of that which gives a full deflection of the instru- 

 ment. This accomplishes the double purpose of making the 

 deflections more nearly proportional to the current, and of 

 eliminating the errors due to magnetic hysteresis, that is, to the 

 tendency of the iron vane to retain its magnetism with a decreas- 

 ing current 



The plunger type of instrument is used more than any other 

 for switch-boards, especially for alternating currents. As a direct- 

 current instrument it has the disadvantage that the direction of 

 its deflection is not reversed by a reversal of the current. 



90. Voltmeters. The voltmeter is a high-resistance ammeter, 

 the scale of which, instead of giving the value i of the current 

 flowing through the instrument, gives the value of the electro- 

 motive force ri between the terminals of the instrument, r being 



