RECENT PROGRESS IN PHYSICS 425 



Casselmann, in his treatise already mentioned, in § 56, gives ex- 

 periments on this subject. If, with opposite horizontal carbon points, 

 the current was passing — 



From The deviation of the apex was to wauls 



N. to S. E. • 



W.toE. N. 



S. to N. W. 



E. toW. S. 



This can be easily deduced. In fig. 61 a and b represent the two 

 horizontal carbon points between which the Pi„ gi 



arc is produced. If now we imagine a per- 

 pendicular plane passed through a and b, 

 and a straight line to pass perpendicularly 

 through the plane between these points, as 

 indicated by the arrow, then a steel needle 

 placed in this line would be magnetized by 

 the current of the arc, and its N. end would 

 be at the point of the arrow when the posi- 

 tive current is passed from a through the arc towards h. But by the 

 influence of the terrestrial magnetism the N. end of the needle would 

 dip, and in like manner also the arc will be inclined from the vertical 

 plane towards the direction of the N. end of the needle. 



If a is to the west, and b east, the inclination will be toward the 

 north when the current is passing from a to b ; but with a direction 

 of the current from east to west, the north end of the supposed mag- 

 netic needle would be on the south side of the arc, and the latter, 

 therefore, would incline toward the south. 



By means of this supposed magnetic needle we can, under all cir- 

 cumstances, determine in what manner the arc will be affected by 

 terrestrial magnetism or either pole of a magnet, or what must be its 

 position when placed between the two poles of a horse-shoe magnet. 



If, instead of one of the carbon poles, a magnetic bar is used, so that 

 the arc is formed between carbon and steel, the arc rotates around the 

 magnetic pole according to the same laws which apply when a mova- 

 ble current rotates around a fixed magnet. The first notice in refer- 

 ence to this rotation of the arc is given by Walker, in the " Transac- 

 tions of the London Electrical Society" from 1837 to 1840.— (Pog. 

 Ann., LIY, 514.) De la Rive also has made experiments on the in- 

 fluence of magnetism upon the voltaic arc, but in a different way. 

 Their description is found in the memoir mentioned already in § 64. 



I shall quote here from De la Rive's memoir literally, in order to 

 give a characteristic example of his want of precision in writing, by 

 which his papers are frequently rendered obscure, as before men- 

 tioned : 



" If two points of soft iron, acting as electrodes, be botb placed 

 within a helix formed of thick copper wire of several coils, the voltaic 

 arc developed between the two points of iron ceases the moment a 

 strong current is passed through the wire of the helices, and reappears 

 if this current be arrested before the points have become cold. The 

 arc cannot be formed between the two iron points when they are mag- 

 netized, whether by the action of the helices or by that of a powerful 



