I860.] Magnetic Force on the Electric Discharge. 171 



and brought to clear definition upon the screen. The negative light 

 above referred to arranges itself, according to Pliicker, in a similar 

 manner. 



5. The rotation of an electric current round the pole of a magnet, 

 discovered by Mr. Faraday in the Royal Institution, nearly forty 

 years ago, was next shown ; and the rotation of a luminous current 

 from an induction coil in an exhausted receiver by the same magnet 

 was also exhibited, and both shown to obey the same laws. This 

 beautiful experiment was devised by De la Rive. 



6. Into a circuit of 20 cells a large coil of copper wire was intro- 

 duced, and when the current was interrupted, a bright spark, due to 

 the passage of the extra current, was obtained. The brightness and 

 loudness of the spark were augmented when a core of soft iron was 

 placed within the coil. The disruption of the current took place 

 between the poles of an electro-magnet; and when the latter was 

 excited, an extraordinary augmentation of the loudness of the spark 

 was noticed. This effect was first obtained by Page, and was for a 

 time thought to denote a new property of the electric current. 



But Rijke had shown in a paper, the interest of which is by no 

 means lessened by the modesty with which it is written, that the effect 

 observed by Page is due to the sudden extinction of the primary spark 

 by the magnet ; which suddenness concentrates the entire force of the 

 extra current into a moment of time. Speaking figuratively, it was the 

 concentration of what, under ordinary circumstances, is a mere push, 

 into a sudden kick of projectile energy. 



7. The contact-breaker of an induction coil was removed, and a 

 current from five cells was sent through the primary wire. The ter- 

 minals of the secondary wire being brought very close to each other, 

 when the primary was broken by the hand, a minute spark passed 

 between the terminals of the secondary. When the disruption of the 

 primary was effected between the poles of an excited electro-magnet, 

 the small spark was greatly augmented in brilliancy. The terminals 

 were next drawn nearly an inch apart. When the primary was broken 

 between the excited magnetic poles, the spark, from the secondary 

 jumped across this interval, whereas it was incompetent to cross one- 

 fourth of the space when the magnet was not excited. This result was 

 also obtained by Rijke ; who rightly showed, that in this case also the 

 augmented energy of the secondary current was due to the augmented 

 speed of extinction of the primary spark between the excited poles. 

 This experiment illustrated in a most forcible manner the important 

 influence which the mode of breaking contact may have upon the 

 efficacy of an induction coil. 



The splendid effects obtained from the discharge of RuhmkorfTs 

 coil through exhausted tubes were next referred to. The presence of 

 the coil had complicated the theoretic views of philosophers, with 

 regard to the origin of those effects ; the intermittent action of the 

 contact-breaker, the primary and secondary currents, and their mutual 

 reactions, producing tertiary and other currents of a higher order, had 



