on the Polarity of the Diamagnetic Force, 165 



ment similar to that made use of by Weber himself; I therefore 

 resolved to write to the constructor of his apparatus, but pre- 

 vious to doing so the thought occurred to me of writing to 

 M. Weber, to inquire whether his further reflections on the 

 subject had suggested to him any desirable modification of his 

 first instrument. In reply to my question, he undertook to devise 

 for me an apparatus, surpassing in delicacy any hitherto made 

 use of. The design of M. Weber was ably carried out by 

 M. Leyser of Leipzig ; and with the instrument thus placed in 

 my possession, I have been able, not only to verify the experi- 

 ments of M. Weber, but to satisfy the severest conditions pro- 

 posed by those who saw in the results of these experiments the 

 efifects of ordinary induction. 



Description of Apparatus. 



A sketch of the instrument made use of in the present inves- 

 tigation is given in fig. 2. BO, B'O' is the outline of a rectan- 

 gular box, the front of which is removed so as to show the appa- 

 ratus within. The back of the box is prolonged, and terminates 

 in two semicircular projections, which haye apertures at H and H'. 

 Stout bolts of brass, which have been made fast in solid masonry, 

 pass through these apertures, and the instrument, being secured 

 to the bolts by screws and washers, is supported in a vertical 

 position, being free from all disturbance save such as affects the 

 foundations of the Royal Institution. All the arrangements 

 presented to the eye in fig. 2 are made fast to the back of the 

 •box, but are unconnected with the front, so as to permit of the 

 removal of the latter. WW are two boxwood wheels with 

 grooved peripheries, which permit of motion being transferred 

 from one wheel to the other by means of a string ss'. Attached 

 to this string are two cylinders, mn, opy of the body to be exa- 

 mined : in some cases the cylinders are perforated longitudinally, 

 the string passes through the perforation, and the cylinders are 

 supported by knots on the string. HE, H'E' are two helices of 

 copper wire overspun with silk, and wound round two brass 

 reels, the upper ends of which protrude from H to G, and from 

 H' to G'. The internal diameter of each helix is 0*8 of an inch, 

 and its external diameter about 1*3 inch; the length from H to 

 E is 19 inches, and the centres of the helices are 4 inches apart; 

 the diameters of the wheels WW being also 4 inches. The 

 cross bar GG' is of brass, and through its centre passes the screw 

 R, from which depends a number of silk fibres which support an 

 astatic arrangement of two magnets, the front one of which, SN, 

 is shown in the figure. An enlarged section of the instrument 

 through the system of magnets is shown in fig, 4. The magnets 



