380 



HON. R. J. STRUTT ON THE LEAST POTENTIAL DIFFERENCE 



glass stopcocks, b is a three-way glass stopcock. I is the manometer tube. A 

 barometer tube, m, is arranged by the side of it, and dips into the same mercury 

 vessel n. The height of the mercury columns in these tubes can be read off to the 

 -^-th of a millim. on the mirror glass scale behind them. The difference of these 

 heights, of course, gives the pressure of gas in the apparatus. 



Fig. 2. 



k is a U-shaped tube immersed in a glass beaker, and serves for the introduction of 

 gas into the apparatus. Before filling with gas, it is necessary to remove the air 

 from the sparking vessel. 



