Sir James Cockle on Transformation. 357 



to introduce a resistance in the primary circuit. But this 

 proved to be wanting in the delicacy required ; and it was 

 found necessary to operate directly on the secondary. Mr. 

 De La Rue had, it is true, already constructed a platinum- 

 wire -resistance of 1,000,000 ohms; but even this amount was 

 insufficient for our present purposes, while its cost was almost 

 prohibitory. Columns of acidulated water proved very effec- 

 tive; but they were open to the objections, first, of heating 

 during use, and, secondly, of evaporation of the water at all 

 times. Both of these actions tended to diminish the resistance 

 of the column. 



Mr. De La Rue had also constructed a resistance, consisting 

 of a thin film of plumbago spread upon ebonite ; but the cur- 

 rent heated the plumbago to such an extent that the ebonite 

 was burnt. The instrument in question was therefore never 

 used. Mr. Ward suggested the employment of slate instead 

 of ebonite ; and accordingly he constructed the instrument 

 actually in use, and represented in fig. 2. The slab of slate is 

 about two feet in length and two inches in breadth. It is 

 fixed at an angle of about 30° to the horizon, the inclination 

 being so adjusted that a block of lead, which serves as a con- 

 tact piece, will slide freely down the inclined plane. A fine 

 wire attached to the block, and passing over a pulley at the 

 upper end, regulates the length of the plumbago actually in 

 circuit. It is found most convenient to use this instrument 

 (which proves to be one of great delicacy) as a shunt. 



A more convenient form of this instrument consists of a 

 vertical rod of ground glass, porcelain, or other incombustible 

 nonconductor, having its surface slightly roughened so as to 

 hold the plumbago. One terminal consists of a double slider 

 capable of being elevated or depressed, and making good con- 



54 



XLIV. Note on Transformation. 

 By Sir James Cockle, M.A., F.R.S., F.E.A.S., $c* 



IN my recent papers (see the Numbers for September 

 1881 and for January last) the details of calculation 

 are kept within narrow limits. There are, however, various in- 

 dependent sources of verification, viz. substitution, reducibility 

 of radicals, comparison of formulas with other results and with 

 each other, and, finally, an actual transformation. 



55. In art. 2 put e = a; the formulas of arts. 8, 9 hold for all 

 values of n. Put a, f h = e, g, k ; the like is true, and the 

 * Communicated by the Author. 

 Phil. Mag. S. 5. Vol. 13. No. 82. May 1882. 2E 



