Rood — Experiments on High Electrical Resistance. 285 



Aet. XXYIII. — Experiments on High Electrical Resistance: 

 Part I ; by Ogden N. Eood, Professor of Physics in 

 Columbia University. 



IjST making electrical measurements of high resistance, if the 

 " direct deflection method " is used, under ordinary circum- 

 stances a resistance of 5000 megohms can be measured with 

 some degree of certainty, and if the conditions are very favor- 

 able, 50,000 megohms maybe reached. The " loss of charge 

 method " is limited by the insulation of the condenser employed, 

 this insulation itself being equal only to about 10,000 megohms. 



In the method describee! in this paper 5000 megohms is 

 small resistance, and it is not till 15,000 or 20,000 megohms 

 has been reached that the manipulation becomes particularly 

 simple. After that, the only limit is found when the resist- 

 ance is so great that the electrometer contrived by me refuses 

 to give any indications, even with electricity of very high 

 potential Just what this limiting resistance is, I cannot at 

 the present moment state, being cut off by the dampness of 

 the air from such determinations ; meanwhile, it is certain that 

 the figure is quite high. 



In the older methods of measuring resistance, a flowing 

 current of electricity is made to act on a system of astatic 

 needles, but in the method here proposed the electricity is 

 allowed to slowly accumulate, till it reaches a certain poten- 

 tial : in the older methods the electricity is treated as though 

 its velocity were infinite, but the present plan is based on the 

 fact that when the resistance is very great the electricity may 

 require ten minutes or even an hour before raising the poten- 

 tial of the electrometer to the required degree. Thus, while in 

 the older methods of work the electricity in its flow resembles 

 a quick-moving incompressible fluid, in the one here described 

 it may be compared to that of a thick pitchy fluid, moving 

 slowly in a long open tank, and attaining its level at the 

 farther end of the tank after the lapse of a longer or shorter 

 time. After its level has been attained, if a leak is made any- 

 where in the floor of the tank, over this leak there will be a 

 depression of the fluid ; if the hole is made in the middle of 

 the tank, some of the fluid drawn off and the hole then 

 stopped, the pitchy fluid will flow to the depression from both 

 sides, but it may take minutes before the old level is once 

 more established. 



Again, if we imagine the walls of the tank to be cold and 

 the pitchy fluid to be at a higher temperature, some of it will 



