92 
Mr. Christie on the mutual action of 
On the law of the variation of the magnetic forces generated at 
different distances during rotation. 
I have already stated, that with the view of determining this 
law from experiment, I made use of a ring instead of a disc. 
This ring is of cast copper, circular, and of uniform thickness ; 
its weight and dimensions as follow : weight 32-37 5 ounces 
troy ; inner diameter io-o inches ; outer diameter 11*9 inches ; 
thickness -24 inch. As I was desirous of avoiding using a file 
on it, the copper was simply cleared from the mould, and the 
small inequalities on its surface would consequently give all 
the measures in excess, so that its specific gravity is higher 
than 8-0954, which these dimensions would give. The mode 
which I adopted for suspending the ring was this : — an assem- 
blage of four small wires (No. 18.) was fixed at the extremi- 
ties of a diameter, so that when stretched, the middle was 
about two inches above the ring, and another similar assem- 
blage fixed in the same manner at the extremities of the 
diameter at right angles : these were made to cross through 
the stirrup at /, as I have before described, so that there 
could be no play in it; that is, the ring could not be turned 
either way without immediately twisting the suspending wire. 
A double wire of the size No. 22, was attached to the stirrup, 
and after the ring had been suspended by it for several hours , 
its length was found to be 46-25 inches. To the ring, two 
pieces of wood, having small slips of brass projecting for 
indexes, were attached diametrically opposite to each other: 
these, with the wires fixed to the ring, made the whole 
weight supported by the suspending wire 32-56 ounces troy. 
The magnets before made use of, the dimensions of which 
