On the Magnetic Effect of Electrical Convection. 179 

 Or, replacing the reciprocals of b and a by their conductivities 



dk b = ck a , 



le --c 



Hence the required conductivity k b is given in terms of the 

 slide- wire reading, multiplied by a constant k a /d. 



It is necessary that the contact between c and d should be 

 invariable. The writer finds that the best contact is attained 

 by a device resembling a drill-bow: the slider carries a small 

 brass bow strung with one or more platinum wires, each of 

 which passes once round the slide-wire. Vertical stops fixed 

 to the slider prevent the platinum wires from lagging behind 

 the bow as it is moved to right or left. 



To graduate the slide-wire scale so that it shall read directly 

 in conductivities, two values of b must be determined by any 

 of the ordinary methods ; these wires can then be put into 

 position successively in the conductivity-bridge, and their 

 balance positions marked on the scale. The whole length of 

 the scale can then be divided in corresponding proportion, in 

 equal divisions. The length of the equal divisions into which 

 the scale is divided depends upon the diameter of the wire 

 under test. It is convenient to be able to vary the length of 

 these equal divisions, and also to be able to move the zero to 

 right or left along the slide-wire. This can best be done by 

 a parallel-ruler arrangement, the straight edges being in a 

 vertical plane through the slide-wire. If the top straight- 

 edge is then divided into equal parts, plumb-lines from those 

 divisions divide the slide-wire into equal parts of any required 

 length ; and the parallel-ruler can be moved, as a whole, along 

 the wire. The angle at which the straight-edge is set depends 

 upon th e diameter of the wire under test, the length being 

 constant ; the apparatus may therefore be calibrated in dia- 

 meters as well as in conductivities. 



XIV. On the Magnetic Effect of Electrical Convection. 

 By Harold Pender*. 



Historical Review. 



FROM a series of experiments extending over the last four 

 years, M. V. Cremieu f has come to the conclusion 

 that a moving electrified body produces no magnetic effect. 



* Communicated bv Prof. J. S. Ames. 



t C R. t. cxxx. p. 1544 (1900); t. cxxxi. p. 578 & p. 797 (1900) ; 

 t. cxxxii. p. 327 & p. 1108 (1901). 



N2 





