494 Mr. J. D. Fry on a New Micromanometer. 



dust. PI. VI. fig. 4 A shews the arrangement of apparatus. 



A disk of very fine wire gauze is supported upon an ebonite 



rod, and the streams of dust directed ao-ainst it from the bottle. 

 * © 



The wire gauze is connected to an electroscope. 



Summary. 



(1) Nearly all kinds of finely divided material when blown 

 into a cloud of dust by a current of air, give rise to electrical 

 charges upon the dust and upon the air. 



(2) The nature of the charge resident upon the dust 

 particles depends upon the chemical characteristics of the 

 material. 



(3) In general, the charge obtained upon the dust is 

 opposite to that associated with the " ion " of the same 

 substance when in solution, i. e. strongly basic bodies give 

 negatively charged dusts, and strongly acidic bodies give 

 positively charged dusts. 



(4) In the case of salts the charge apparently depends on 

 the relative strengths of the acidic and basic ions. 



(5) Similarly constituted bodies give similar charges. 



LIII. A Neio Micromanometer. By J. D. Fry, Lec- 

 turer and Demonstrator in Physics in the University of 

 Bristol *. j 



I 



^I^HE following is an account of a new micromanometer 



JL which has been designed by the author and which is 

 now in use in the Physics Department of the University of 

 Bristol. 



The principle of the instrument is simple. The difference 

 of pressure to be measured is applied to the two sides of a 

 stretched circular membrane. It is arranged that the centre 

 of the membrane by its displacement twists a mirror which 

 is suspended in a special manner, the pressure difference 

 being finally indicated by the deflexion of a spot of light on 

 a scale. 



It is clear that with certain slight modifications of con- 

 struction a manometer working on this principle may be 

 adapted to the measurement of small differences of pressure 

 at any absolute pressure. The form herein described has 

 been designed for use at atmospheric pressure. The con- 

 ditions governing the sensitiveness of a gauge of this type, 



* Communicated bv the Author. 



