Electric Wind in Hydrogen containing traces of Oxygen. 451 



lower halves. Above M x M 2 are the discharge-tubes D 1 D 2 

 containing the gas to be experimented on. Above M 3 M 4 is 

 water WiWj. 



. W 2 is continued upwards to form a bubble of water, B, in 

 benzene (shaded) at the top of an inverted glass funnel 

 sealed into the tube which contains it. The upper surface 

 of the bubble is focussed on to the cross-wire of the micro- 

 scope A, and forms a very delicate indicator of the small 

 differences of pressure applied between the surfaces of Mj 

 and M 2 . The diameter of the bubble is about 4 mm. 



In making a measurement the bubble is kept on the cross- 

 wire by tilting T with S 3 , and the tilt then affords a measure 

 of the difference of pressure applied*. 



When working well the movement of the bubble is just 

 visible under a pressure difference of 10 ~ 2 dyne cm. -2 



E E are taps for locking the manometer during the pumping 

 out of the discharge-tubes. 



At the bottom of each discharge-tube is a plate of thin 

 rolled sheet zinc covered with perforations (about 500 of 

 them) and connected with a sensitive galvanometer. This 

 plate, shown by a dotted line, is the surface against which 

 the point P discharges. It is more convenient than the ring 

 used in the earlier wind-pressure work, as it does not entail 

 so long a discharge-tube. The perforations must be such 

 that the pressure below the plate is the average of the pres- 

 sures on its upper surface. For this to occur the perforations 

 must all be equal and equally spaced, and the flow of gas 

 (if any) through the plate at each perforation must be pro- 

 portional to the difference of pressure producing it. 



This is not an easy condition to fulfil in general ; but in 

 our case the differences of pressure that could occur between 

 the two sides of the plate were so extremely small that we 

 were able to obtain the requisite proportionality with sufficient 

 accuracy by using the point of a fine sewing-needle suitably 

 supported to make the perforations. 



* This manometer is a modification of the one designed twelve years ago 

 for the original measurements of the electric wind (Ohattock, Phil. Mag. 

 1899, loc. cit.). The latter was subsequently improved by Mr. J. D. Fry, 

 and in this form used and described by i)r. T. E. Stanton (Proc. Inst. 

 C. E. vol. clvi. 1903-4, pt. ii.). In the instrument just described benzene 

 replaces oil as it is less sluggish, and its surface remains clean for much 

 longer periods, provided that the water in contact with the benzene is 

 air free. When, as in our case, it is necessary to trap the water with 

 mercury the effect of the latter on the working of the manometer can be 

 practically got rid of by using large mercury surfaces and putting the 

 mercury tubes at right angles to those containing the water, as indicated 

 in the diagram. The manometer must then be calibrated in terms of a 

 second manometer containing water onlv. 



2 G2 



