180 



Rev. T. R. Robinson on increasing the 



[May 31, 



the inductorium all tension above what is necessary to force the necessary 

 quantity of current through the circuit is useless, nay sometimes injurious*. 

 I am inclined to think that a tension which gives sparks of 4 inches will 

 be found quite sufficient in ordinary cases, and this will be given by about 

 20,000 spires ; all beyond only adding to the weight of the instrument, 

 its cost, and the difficulty of insuring its insulation. It must be kept in 

 mind that the mere quantity is independent of the length 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 11 : 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 11 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 quan- 

 tity within the limits which I tried ; their length seems to increase 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 im- 

 pression on the eye shall be continuous. 



The quantity increases with the diameter of the wire up to a maximum, 

 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 powersf . 



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 result- 

 ing 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, 

 hut are much denser, and in the higher combinations 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 re- 

 sistance ; thus the arrangement G . K gives little less than the sum of its 



* It has often been remarked that intense discharges will not show strata well in an 

 exhausted tube. 



t The connectors add some resistance and some counter-induction. 



