230 Mr, W. Makower on a Determination of the 



in the gauge made contact with the wire ic, the contact e 

 was disconnected from / and connected to g, thus cutting- 

 out the Wheatstone-bridge and placing the gauge-key N 

 in series with a storage-cell IS and a chronograph. The 

 platinum point p, which was electrically connected to w, 

 was brought just into contact with the top of the mercury- 

 column, when the air in the "air-vessel" was adjusted to the 

 initial pressure (p'ij. On releasing the pressure the chrono- 

 graph circuit was broken at p and made again through the 

 wire w } after the expiration of a certain time (depending 

 on the rate at which the mercury fell) which was measured 

 on the chronograph to about "01 second. 



This time was varied from *5 second up to about 5 seconds. 



4. Pressure Measurements. 



The excess pressure (pi— p 2 ) in the " air-vessel " before 

 opening to the atmosphere was measured on a manometer M 

 rilled with Fleuss pump-oil. The density and coefficient of 

 expansion having been carefully determined the excess 

 pressure could he obtained in centimetres of water by means 

 of the formula 



density = '8826- "000644 £ 



(where t = temperature centigrad 



* 



As the oil used w T as exceedingly viscous some trouble was 

 experienced at first, owing to the long time taken by the oil 

 in running down the sides of the tube when the pressure was 

 altered. For this reason the position of the oil in the mano- 

 meter was prevented from shifting more than two or three 

 centimetres by closing the tap E immediately before opening 

 the stopper b. 



The pressure (p.,) was obtained by reading the barometer. 



5. Ohservations. 



The following is a series of observations. 



The resistances are given in arbitrary units, of which 

 100 = 1*31 ohms approximately. 



The kicks of the galvanometer-needle are given in terms 

 of the micrometer-divisions in the eyepiece of the reading 

 microscope. 



By plotting K against r the change of resistance corre- 

 sponding to no kick of the galvanometer is found to be 10'47. 



* The density and coefficient of expansion of the oil were determined 

 by Mr. N. Eumorfopoulos, of University College, who very kindly 

 supplied me with the oil used in these experiments. 



