298 



ELEMENTARY LESSONS ON [CHAP. v. 



shown in Fig. 125. The inner coil CD is suspended with 

 its axis at right angles to that of the outer coils AA, BB, 

 and is supported bifilarly (see Art. 118) by two fine 



metal wires. If 

 one current flows 

 round both coils in 

 either direction the 

 inner bobbin tends 

 to turn and set its 

 coils pa'rallel to 

 the outer coils ; 

 the strength of 

 the current being 

 measured by the 

 sine of the angle 

 through which the 

 suspending wires 

 | are twisted. The 

 chief advantage 

 of this instrument 

 over a galvano- 

 meter is, that it 

 may be used for 

 induction-currents in which there are very rapid alter- 

 nations, a current in one direction being followed by a 

 reverse current, perhaps thousands of times in a minute. 

 Such currents hardly affect a galvanometer needle at all, 

 because of the slowness of its swing. 



337. Electromagnetic Actions of Convection 

 Currents. According to Faraday a stream of particles 

 charged with electricity acts magnetically like a true con- 

 duction-current. This was first proved in 1876 by 

 Rowland, who found a charged disc rotated rapidly to 

 act upon a magnet as a feeble circular current would do. 

 Convection currents, consisting of streams of electrified 

 particles, are also acted upon by magnets. The convec- 

 tive discharges in vacuum-tubes (Art. 292) can be drawn 



Fig. 125. 



