the Mercury Unit of Electrical Resistance, 



165 



Kg. 1. 



is shown in fig. 1, in which B is a screw 

 carrying a round plateE of polished iron 

 on a ball-and-socket joint, F a piece of 

 india-rubber pipe by which the glass 

 tube is held fast in the frame, and f, g 

 screws for putting the frame together. 

 On the opposite end was fixed a rect- 

 angular bent piece of glass tube 20 

 millims. diameter, held tightly on the 

 tube by means of an india-rubber stop- 

 per. The tube was placed horizontally 

 on a table and filled very slowly. If 

 no bubbles could be seen in the tubes, 

 the plate E was pressed tightly by 

 turning the screw B against the end. 



The rectangular bent glass was then 

 removed, and, leaving a small globe of 

 mercury protruding from the opening 

 of the tube, the whole was hung up 

 with a delicate thermometer for an 

 hour in the open air, protected by 

 screens from radiant heat. 



The temperature was then read off, 

 and the protruding hemisphere of mer- 

 cury removed by pressing a plate of ground glass on the top of 

 the tube. 



The mercury was allowed to run very slowly out into a little 

 porcelain crucible, so as to leave no globules behind, and weighed. 



In the following Table, column 2 gives the temperatures; 

 column 3 the weights (in air of mean temperature) in grammes, 

 after deducting the weight of the crucible ; column 4 the same 

 reduced for expansion of glass and mercury and for vacuum to 

 0° C.j column 6 the lengths, and column 7 the average inner 

 radius. 



The formula by means of which the weights were reduced is 



P =^{1 + (Y-<7)*} 1*00009, 



and is correct within the limits of their measurements. 



y = coefficient of cubic expansion of mercury, per degree 

 Centigrade =0-00018018. 



g= the same of glass = 000002586. 



I have taken (<r) the specific gravity of mercury atO°C. = 13*557. 

 The constant 1*0000908 reduces the weights for vacuum. 



The lengths of the tubes were measured on the brass metre 

 scale of our resistance bridge. 



