226 MR J. C. BEATTIE ON THE 



The following experiments were carried on in the Ph}^sicalisches Institut, Muenchen ; 

 and I have to thank Professor Boltzmann for the trouble he put himself to, for his 

 suggestions, and for placing the whole resources of his laboratory at my disposal. 



The plates used were cast from two separate quantities of ordinary mercantile 

 bismuth. In some instances they were cooled quickly, in others slowly. The thicknesses 

 varied from three to one millimetre ; the ratio of length to breadth was about three to 

 one as the plates were originally used ; afterwards these dimensions were considerably 

 modified. 



The galvanometer used was a Wiedemann, with a Siemens well-formed magnet. 

 The electro-magnet used for the creation of the magnetic field consisted of two 

 cylinders of soft iron 60 cm. long, 16 cm. in diameter, placed on a parallelepiped of 

 the same material 63 cm. long, 20 cm. high, 20 cm. broad. The shoes were formed by 

 two blocks 16 cm. square, 20 cm. long, to which truncated cones were fixed with a 

 base diameter of 16 cm., a summit diameter of 6 cm. Each cylinder was surrounded by 

 two spools, round which the copper wires were wound. Diameter of the wire 2" 5 mm. ; the 

 total length of wire was 3850 mm. ; the number of windings 5951. (Cp. fig. A, Plate P.) 

 The current to the electro-magnet was supplied by an accumulator battery of 

 56 cells. 



The strength of the field was measured by Verdet's method. A wire was arranged 

 in the form of a square, the ends were inserted into the galvanometer circuit, and 

 when the electro-magnet was on, the square which was kept perpendicular to the 

 lines of force was pulled quickly out of the field. 



The readings thus obtained were compared with those obtained from an earth 

 inductor inserted in the same circuit, and the strength of the field in absolute units 

 arrived at in the usual way. To get the strength of the field in absolute units, the 

 numbers given as field strengths in the results must be multiplied by 138 "5. 



The strength of the current which flows in the direction of the plate's length — and 

 which will be called the primary current — w r as measured at the beginning and end of 

 each series of experiments. For this purpose a thick copper wire was inserted in the 

 primary circuit. To two points of this, copper wires were soldered, which, by means of 

 a commutator could be placed in the galvanometer circuit when necessary. 



The electro-magnet was so placed that it exercised a minimum effect on the galvano- 

 meter, which was at a distance of thirty or forty feet. The magnet and primary 

 currents could both be reversed by commutators ; the number of readings necessary to 

 eliminate disturbing effects was thus four. The average of the four readings was divided 

 by the primary current strength : this quantity is called later the transverse effect. 



The positive direction of the transverse effect is so defined : Let the plate of bismuth 

 be supposed to be in the plane of the paper with the north pole of the magnet above, the 

 south below, the paper. Then, if in going from the point where the primary current 

 enters to that where the transverse current enters the motion is counter clock, we call the 

 transverse effect positive. 



