2?S ELEMENTS OF ELECTRICITY AND MAGNETISM. 



coil to be deflected. The plunger type of ammeter is extensively 

 used where cheapness is a prime consideration. In instruments 

 of this type the current flows through a coil of wire which mag- 

 netizes and moves a pivoted or suspended piece of soft iron to 

 which the pointer is attached. 



MEASUREMENT OF RESISTANCE. 



152. Absolute measurement of resistance. Lorenz's method. A circular disk 

 of copper DD, Fig. 204, is mounted on an axle and driven at a uniform speed of 



pulley 



D- 



n 







s 



Fig. 204. 



revolutions per second. This disk is surrounded by a large coil, or solenoid, SS 

 through which a steady current / (the value of which need not be known), flows 

 from a battery B. This current also flows through the resistance R which is to be 

 measured, and an auxiliary circuit containing a sensitive galvanometer G is connected 

 so that the electromotive force which is induced in the rotating disk between the 

 brushes a and b can be balanced against the electromotive force RI across the re- 

 sistance R t this balance being indicated by zero deflection of the galvanometer. The 

 intensity of the magnetic field in the solenoid SS is ff= 47727, everything being 

 expressed in c.g.s. units and 2 being the number of turns of wire per centimeter 

 length of the solenoid. Any given radial filament of the rotating disk cuts rrr* X H 

 lines of flux during each revolution of the disk, so that the electromotive force induced 

 between the center and the circumference of the disk is equal to rrr 2 X -^X n or 

 Trr* X 4 1I " 3 ^'X ** When the speed of the disk is increased until the galvanometer 

 gives no deflection, then this induced electromotive force is equal to RI, whence we 

 have 



R = 



153. Resistance boxes. The measurement of resistance ordi- 

 narily consists of the determination of a given resistance in terms 

 of a known resistance. In many cases this measurement is ac- 

 complished by adjusting a known resistance until its equal to the 

 resistance to be measured, and a resistance box is an arrangement 



