826 Mr. F. B. Young on the 



copper disk to the top of which is cemented a piece of thin- 

 walled glass tubing, constricted near the bottom. On the 

 top of the copper float is soldered a piece of platinum-foil ; 

 the under surface is amalgamated. When contact is made, 

 a rela}* Gr is actuated and a current of about 1 ampere is sent 

 through a 5-ohm coil of fine german-silver wire H contained 

 in the ballon B and lightly wrapped in cotton-wool. The 

 heat developed causes the air to expand until the consequent 

 rise of pressure breaks the contact at E. If the temperature 

 of the ballon is already rather higher than that of the atmo- 

 sphere, the pressure again falls owing to radiation of heat, 

 until contact is again made. The cotton wool has the effect 

 of making the expansion less sudden, so that the mercury in 

 the gauge may follow the changing pressure more closely. 

 Since the height of the mercury in C can be regulated by 

 means of the reservoir D, any desired pressure can be main- 

 tained in the apparatus. 



A slight tendency to stick on the part of the contact E is 

 corrected by means of the electromagnetic trembler K, which 

 is placed as a shunt in the heating circuit. Its base-board is 

 attached to the needle F by a wire whose tension is suitably 

 adjusted. The tendency is reduced to a minimum by deli- 

 cately poising the relay key and reducing the relay current 

 to a minimum. 



Tests have shown that the contact can be relied upon to 

 make and break within a range of 0*035 mm. movement of the 

 float, the maximum oscillation of pressure due to this cause 

 being therefore 0'07 mm. of mercury ; in general the oscil- 

 lation is less than this. The troublesome effects due to the 

 surface tension of the mercury are practically eliminated by 

 making the manometer tubes rather wide (about 2 cm.), and 

 by introducing a thin layer of ' Fleuss ' oil over each mercury 

 surface. It is also advisable to avoid depressing the float 

 into the mercury during the process of adjustment. 



The manometer is made independent of temperature varia- 

 tions by a suitable adjustment of its dimensions. Let P be 

 the indicated pressure of mercury, S the area of cross-section 

 of the longer limb, V the total volume of mercury, c the 

 coefficient of cubical expansion of mercury, and a the coeffi- 

 cient of linear expansion of glass. Then, assuming the float 

 to remain unmoved relatively to the glass tube, the change of 

 pressure dp due to a change of temperature dt is given very 

 approximately by 



dp = Y(c -^- dt +a.F.dt-c.P.dt. 



