LUMINOSITY OF FLAMES CONTAINING SALT VAPOURS. 
67 
the flame were led out of the room. The lower end of the tube, C, was fitted with 
a brass disk having a hole in it about 4 cm. in diameter, which was covered with 
a platinum wire grating like the one above the burner, B. At P a fine platinum 
wire was stretched horizontally across the flame perpendicular to the plane of the 
paper. This wire was carried by a glass tube which was supported on a slide, EE', 
so that the wire could be moved up and down along the vertical axis of the flame. 
The slide was provided with a millimetre scale and vernier. The potential difference 
between the wire, P, and the burner grating could be measured with an electrostatic 
voltmeter. 
The tube, C, and the upper grating were usually connected to the earth, through 
a galvanometer, and the burner, B, and lower grating could be connected to a 
battery of 1800 dry cells, giving up to 2700 volts. In this way a vertical electric 
field could be maintained in the flame between the upper and lower gratings. 
Let v denote the upward velocity of the flame, n the number of puffs of salt 
entering the flame per second, x 1 the height of a puff as seen through the rotating 
disk and x 2 the height of the next puff higher up. In the absence of an electric field 
the puffs simply move up with the flame so that 
v — n (x 2 —x 1 ). 
The heights of the puffs could be measured by adjusting the wire, P, so that it 
coincided with their tops or bottoms and reading the vernier on the slide, EE'. 
About three puffs were visible when the number of puffs was rather over 100 per 
second. 
The puffs were equidistant showing the velocity of the flame to be sensibly uniform. 
It was found that x 2 —x x was nearly 3 cm. when N was equal to 100 per second. 
The velocity of the flame was therefore about 300 cm. per second and could be 
determined to within two or three per cent, without difficulty. The gas and air 
supplies to the flame were carefully regulated, and the gas and air were well mixed 
before they entered the burner. The burner gave an exceptionally steady and only 
slightly conical flame in the space between the wire gratings. 
Let lc denote the velocity of a luminous salt molecule in the flame due to an electric 
field of unit strength. We have 
« 
dx' = (v — kX.) dt, 
where x' denotes the height of the molecule above the lower grating, X the electric 
force reckoned positive when directed downwards, and t the time. Also in the 
absence of an electric field let 
dx — v dt, 
where x denotes the height of the molecule in this case. 
