174 



Rev. T. R. Robinson on increasing the 



[May 31, 



The potential 11 is as n"^ multiplied by another function of the length and 

 diameters of the helix (see Maxwell's valuable paper On the Electromag- 

 netic Field/' Phil. Trans. 1865) ; and the term h is always less than unity. 

 When helices are consecutive their 11' s are added, not when collateral. 



From this it follows that may be neglected ; that — ^ is nearly 



jU — O fl — 



unitj ; and that the difference between F and $ increases as b diminishes. 



When equal helices are consecutivCj b as well as F are unchanged ; there- 

 fore so is ^. 



When they are collateral, each separate b remains unchanged (unless 

 they be so close that they react on each other) ; and therefore, as with F, 

 the resultant 4> is the sum of its components. 



If the resistance of the wire be diminished by increasing its section with- 

 out making much change in the dimensions of the helix, b is diminished, 

 and therefore the coefficient of F. It is evident from the form of equation 

 (c) that has a maximum for r, and that beyond this there is actual loss 

 of power in increasing the thickness of the wire. 



It remained to test these views by experiment, but the task has some 

 difficulties. A single discharge of inductive electricity is usually deter- 

 mined by the swing which it causes in a galvanometer needle ; but it is 

 scarcely possible to get two discharges exactly equal. The slightest varia- 

 tion in the manner of breaking the circuit, the least oxidation or roughen- 

 ing of the surfaces where the break is made, change the result ; and there- 

 fore it seemed best to take the actual working of the inductorium, in hopes 

 that the average of some thousand discharges must be near the real value 

 of the current. 



The rheometer which I used is Weber's (for the use of which I am in- 

 debted to the kindness of Mr. Gassiot), and it showed an amount of fluc- 

 tuation even greater than I expected. With every precaution as to the 

 action of the rheotome, the mirror of the Weber never becomes stationary, 

 and the oscillations are irregular ; twelve of them were taken for each set, 

 of course read at each end and reduced by the usual formula ; yet the sets 

 differ so much, that I only offer their results as tolerable approximations. 

 Two facts illustrating this uncertainty may be mentioned. With a me- 

 chanical rheotome driven at a uniform speed, and its acting surfaces plati- 

 num, the ratios of the current were — 



When set so that the point rises but little from the anvil . . TOOOO 



Rise greater , ]'7S94 



Rise still greater, tension of spring greater 1*9685 



Rise still greater, tension further increased , 1*83/1 * 



Here a shght change of the adjustment nearly doubles the action of the 

 inductorium. 



Another cause of uncertainty is the variable speed of the rheotome. In 

 general it is worked by the primary current, and therefore is affected by 

 fluctuation of the battery and the extra current of the primary. The me^ 



