Intelligence and Miscellaneous Articles. 157 



presented by the form (1) ; but the equation (1) completely repre- 

 sents the formula given by me*. If, therefore, Mr. Lodge consi- 

 ders the equation (1) correct, I really cannot understand how mine 

 can be erroneous. 



As to the point of view from which I tried to explain the 

 origin of the empirical formula more than thirteen years ago, 

 the latter, after the results of the experiments of Le Roux were 

 added to those of Thomson, was subject to changes — a fact which 

 I positively expressed in my paper, " Ein Beitrag zur Theorie der 

 Thermostrome." 



In 1S70 I first ga,ve a theoretical proof of the truth of my empi 

 rical formula (equation 1) in the 'Reports' of the University of 

 Kiew, and another in 1873, in Pogg. Ann. vol. cxlix. If Mr. Lodge 

 had been acquainted w T ith these memoirs, he certainly would not 

 have reproached me with not taking notice of Thomson's two effects. 



ON AN ARRANGEMENT FOR REPRODUCING, WITH THE AID OF 

 THE SIREN, THE EXPERIMENT OF FOUCAULT (ARREST OF A 

 ROTATING DISK UNDER THE ACTION OF AN ELECTROMAGNET). 

 RY M. BOURBOUZE. 



A well-known experiment of Faraday consists in placing between 

 the poles of a powerful electromagnet a cube of copper or silver 

 suspended at the end of a strongly twisted string ; the cube, left 

 to itself, having taken a movement of rapid rotation, if a current 

 be thrown into the electromagnet, the cube stops almost instan- 

 taneously. 



Foucault realized a similar effect on causing a copper disk to 

 rotate between the poles of an electromagnet. The mechanism he 

 employed to produce the rotation was that which had served him 

 for putting his gyroscope in motion. 



The arrangement which I have adopted in order to repeat these 

 experiments permits one to render sensible to a large audience the 

 action of magnetism on a rotating disk. I fix, upon the axis of a 

 siren, a disk of copper parallel to the movable plate, and rotating 

 between the armatures of an electromagnet w 7 hich may be carried 

 by the siren itself. To set the siren in motion, I utilize the air- 

 impelling apparatus which serves me for most of my acoustic ex- 

 periments. The apparatus once started, and the sound produced 

 being the sharper the greater the velocity of the rotation, the 

 current is caused to pass into the electromagnet : the plate is 

 arrested, and the sound, till then perceptible at a great distance, 

 entirely ceases. 



I ask permission to give on this occasion some account of the 

 apparatus w r hich I substitute for the bellows usually employed to 

 set the siren going. It consists of a reservoir of compressed air 

 of 50 litres capacity, put into communication with a pipe the 

 extremity of which is very fine, and which penetrates the axis of 

 a much larger conical tube. In the outer tube, as in the Bunsen 



* My formula is E = (T 1 -T 2 )(ft+e(T 1 +T 2 )) ? whereb ^ A = -T re- 

 presents the temperature of the neutral points. 



