164 Dr. J. Hopkinson on High Electrical Resistances. 



tolerably strong lines; its resistance was first found to be 

 209,907 ohms, and four months later to be 208,840. 



b has four strong lines on a strip half an inch wide; resist- 

 ance 207,954 on a first occasion, and 208,750 after the lapse 

 of four months. 



a has two lines narrower than the preceding; resistance 

 5,240,000 at first, and 5,220,800 after four months. 



h has a single line apparently similar to either of those of a; 

 and the resistance is 9,168,000. 



k, I, and m have each two lines ruled with a HHH pencil ; 

 their resistances are respectively 23,024,000, 14,400,000, and 

 13,218,000 ohms. 



c and e also have two lines, but they are finer ; the resist- 

 ances are 79,407,000 and 96,270,000. 



As already mentioned, all the preceding were tested with 

 various battery-power, and were found to obey Ohm's law 

 within the limits of observation. It was not so with/, as the 

 following observation shows very clearly, c, k, e, and / were 

 arranged as a Wheatstone's bridge. Junctions (/, c) and (e, k) 

 were connected to the poles of a Daniell's battery varying 

 from one to eighteen elements; junctions (e,f) and (k, c) 

 were respectively connected through the reversing-key with 

 the quadrants of the electrometer. The potential of one Da- 

 nielPs element was represented by 270 divisions of the scale 

 of the electrometer. Column I. gives the number of elements 

 employed, II. the corresponding reading of the electrometer, 



Jc e 



III. the value of -= -7 — - deduced therefrom, and IV. the 



fc + c f+e 7 



values of the ratio resistance of/: resistance of e. 



in. iv. 



0-060 5-1 



0-046 4-6 



0-039 4-4 



0-029 4-1 



0-021 3-9 



0-017 3-8 



0-0041 3-5 



-0-0016 3-4 



-0-006 3-3 



-0-0097 3-25 



This result is by no means surprising. There is doubtless 

 an exceedingly minute discontinuity in the fine line across 

 which disruptive discharge occurs ; and the moral is, that re- 

 sistances of this kind should always be tested as regards their 

 behaviour under varying electromotive force. 



Several attempts to rule a line on a strip 12 inches long with 

 a resistance over 100,000,000 ohms resulted in failure. 



I. 



II. 



1 



16 



2 



25 



3 



34 



4 



31 



5 



28 



6 



27i 



9 



10 



12 



- 5 



15 



-25 



18 



-47 



