72 Royal Society, 



of wire : I actually found it the same for a flat spiral of 21 spires 

 and for a helix of 13,655. 



It is not, I believe, ascertained what is the best proportion of 

 height and diameter for a secondary helix of a given number of 

 spires. It is generally made as long as its primary, though perhaps 

 not on any definite principle. The magnetic potential P is in this 

 form a little greater than in that which I used, but so also is II : the 

 length of wire is less, which increases F, but also decreases b ; and 

 a priori it is not easy to decide which way the balance inclines. 

 The II is something less if the spires be in separate sections than if 

 they be in one continuous coil. 



The dimensions of the core do not seem to be of importance as to 

 quantity within the limits which I tried ; their length seems to in- 

 crease the tension. 



The quantity is greatly diminished when the rheotome works 

 rapidly ; and in spectral work the probable limit of its slowness is 

 that the impression on the eye shall be continuous. 



The quantity increases with the diameter of the wire up to a maxi- 

 mum, which is attained when this is about the sixty-fifth of an inch. 



Helices may be combined either for tension or quantity without 

 much loss of these respective powers*. 



If for the first, they are combined in series ; the general tension 

 is the sum of the individual ones, and in this way we can obtain 

 sparks of a length limited only by the strength of the insulator 

 which is interposed between the primary and secondary helices. 

 If the latter be all of the same wire, the quantity remains unchanged ; 

 if they differ in this respect, it will be intermediate between the weakest 

 and strongest. 



If they are combined for quantity, they must be set collaterally, 

 i. e. all their positive terminals connected, and all their negative. The 

 resulting current will be the sum of all the separate ones, but the 

 tension is not increased; the sparks seem even a few hundredths of 

 an inch shorter, but are much denser, and in the higher combi- 

 nations approach to the character of a jar discharge. Hence there 

 is no risk to the apparatus by extending this mode of combination to 

 any extent. 



It deserves notice that the helices need not be equal in tension 

 or resistance ; thus the arrangement G . K gives little less than the 

 sum of its components, though K has only half as many spires as G 

 and but a tenth of its resistance. 



In combining these instruments, the primaries should not be con- 

 secutive if of large numbers, for so the action of their extra-current 

 would be very destructive to the rheotome; with P' + P" containing 

 726 spires in series the spark in the mercurial one is almost explo- 

 sive, but when they are collateral it works quietly. "Were, however, 

 ten or twelve to be so combined, it would require a battery of very 

 large cells to maintain the current, and it is better to have a separate 

 battery for each pair of primaries. In this I find no difficulty ; the 

 negative f poles of all the batteries are connected with the mer- 



* The connectors add some resistance and some counter-induction, 

 t If the mercury be made positive, each discharge makes a sharp report and 

 blows about the metal and alcohol in most unpleasant profusion. 



