10 



Mr. S. Bidwell. 



[Feb. 1, 



current increases, the deflection of the galvanometer needle becomes 

 greater until a certain strength is attained, when the galvanometer 

 needle suddenly goes back, and comes to rest almost or exactly at 

 zero. The lighter the pressure the smaller the current necessary to 

 produce this effect. Table VIII gives the currents with which the 

 rise occurred, the pressure at the points of contact varying from *05 

 grm. to 5 grms. 



Attempts were made to measure the high resistance with a Wheat- 

 stone's bridge and a small current, and this brought to light the 

 curious fact that the resistance in question is generally very much 

 lower for weak than for strong currents. Thus it may happen that 

 while the moment before measurement the resistance may be sensibly 

 infinite, ^the very act of measurement reduces it to a few hundred 

 ohms. 



This paradoxical effect is best shown by the special arrangement 

 indicated in fig. 5. 



G is the tangent galvanometer, S the carbon balance in the scale- 

 pan of which is a weight of 1 grm. When key 1 is depressed the 

 current from eight Leclanche cells passes through the galvanometer 

 and the carbons. The immediate effect is a deflection of (in the case 

 of my galvanometer) about 75°. Almost instantly, however, the 

 -needle returns to zero, and key 1 may now be raised and depressed 

 without producing the slightest movement of the needle. If, how- 

 ever, key 2 is depressed the current from a single cell will pass 

 through the galvanometer and the carbons, and this will generally 

 produce a small deflection of from 2° to 10°. But if key 1 is once 

 more depressed the needle again altogether fails to move. The 

 resistance at S is therefore of a very peculiar nature, being more 



