368 ELEMENTARY LESSONS ON [CHAP. x. 



the direction and number of lines -of -force. If a small 

 wire coil be joined in circuit with a long -coil galvan- 

 ometer having a heavy needle, and the little coil be sud- 

 denly inverted while in a magnetic field, it will cut all 

 the lines-of-force that pass through its own area, and 

 the sine of half the angle of the first swing (see Art. 

 204) will be proportional to the number of lines of 

 force cut ; for with a slow-moving needle, the total quan- 

 tity of electricity that flows through the coils will be the 

 integral whole of all the separate quantities conveyed 

 by the induced currents, strong or weak, which flow 

 round the circuit during the rapid process of cutting 

 the lines-of-force ; and the little coil acts therefore as a 

 magnetic proof -plane. 



If the circuit be moved parallel to itself across a uni- 

 form magnetic field there will be no induction currents, 

 for just as many lines-of-force will be cut in moving 

 ahead in front as are left behind. There will be no cur- 

 rent in a wire moved parallel to itself along a line-of-force ; 

 nor if it lie along such a line while a current is sent 

 through it will it experience any mechanical force. 



403. Earth Currents. The variations of the 

 earth's magnetism, mentioned in Lesson XII., alter the 

 number of lines-of-force which pass through the tele- 

 graphic circuits, and hence induce in them disturbances 

 which are known as " earth currents." During magnetic 

 storms the earth currents on the British lines of telegraph 

 have been known to attain a strength of 40 milliwebers- 

 per-second, which is stronger than the usual working 

 currents. Feeble earth currents are observed every day, 

 and are more or less periodic in character. 



404. Self-induction : Extra-Currents. In Art. 

 397 the induction of one circuit upon another was ex- 

 plained, and was shown to depend upon the number of 

 lines-of-force due to one circuit which passed through . 

 the other, the coefficient of mutual induction M being 

 the number of mutual lines-of-force embraced by both 



