Dr. J. Hopkinson on High Electrical Resistances. 163 



much greater (which do not conduct according to Ohm's law, 

 but with a resistance diminishing as the electromotive force 

 increases). A short description of these may perhaps save a 

 little trouble to others who desire tolerably constant high re- 

 sistances. 



All my resistances are ruled on strips of patent plate glass 

 which has been finished with fine emery, but has not been 

 polished. The strips are twelve inches long, and, except in the 

 cases specified below, about half an inch wide. One or more 

 parallel lines are ruled on each strip, terminating at either end 

 in a small area covered with graphite from the pencil. The 

 strip of glass, first heated over a spirit-lamp, is varnished with 

 shellac varnish, excepting only these small terminal areas, 

 which are surrounded by a small cup of paraffin-wax to con- 

 tain mercury to make the necessary connexions. To secure 

 better insulation, feet of paraffin or of glass covered with pa- 

 raffin are attached on the underside at the ends of the strip 

 to support it from the table. Before varnishing, each strip was 

 marked with- a distinguishing letter. The strips marked g, h, i, a, 

 and b were ruled with a BB pencil, the remainder with a HHH. 



These resistances appear to be not quite constant, but to 

 vary slightly with time, the maximum variation in four months 

 being slightly in excess of ^ per cent. In every case they 

 were examined under varying potential to ascertain if they 

 obeyed Ohm's law. With the exception off, described 

 below, all were satisfactory in this respect. The resistance 

 appears to diminish slightly as the temperature rises ; but this 

 conclusion rests on a single rough experiment, and must be 

 regarded as uncertain. 



The values of the resistances were determined with a dif- 

 ferential galvanometer, each coil having a resistance of 3500 

 ohms, by the well-known method of dividing a battery-cur- 

 rent, passing one part through the large resistance to be mea- 

 sured and one coil of the galvanometer, the other through a 

 set of coils or other known resistance, and then through the 

 galvanometer shunted with a second set of resistance-coils. 

 g was thus compared with standard coils, g was then used to 

 find h and i ; and h + i was used to find a and b. A Thomson's 

 quadrant electrometer was used to compare in succession k, I, 

 and m with a + b. c and e were similarly compared with 

 k + 1 + m ; and, lastly, c and e were used to examine /. 



g is ruled on a strip one inch wide, rather more than half 

 the surface being covered with graphite. Three experiments 

 on the same day gave 26,477, 26,461, and 26,470 ohms; the 

 variations are probably due to uncertainty in the temperature- 

 correction, the galvanometer-coils being of copper. After the 

 lapse of four months 26,615 ohms was obtained. 



i is ruled on a strip three quarters of an inch wide, with nine 



