474 Messrs. Haselfoot and Kirkby: Electrical Effects 
of the latter were attached by wires to the poles of a battery of 
two volt accumulators, one of the poles, of course, being put to 
earth. Immediately before the e explosion the quadrants were 
insulated, and on the explosion taking place the electrometer- 
needle was brought back to its zero by moving the slider 8 
of the potentiometer from the point of zero potential along 
the potentiometer-scale. When this has been done the 
potential of one terminal of the condenser is zero, of the 
other, V, the potential shown by the potentiometer. The 
charge on the condenser is therefore CV, where C is the 
capacity of the condenser, and this is the quantity of elec- 
tricity thrown on to the wire W. ‘This method greatly 
extended the range of the electrometer, which was of the 
Dolezalek pattern, in measuring quantity. Moreover, by 
thus employing a null method the leak of electricity on the 
electrometer was rendered negligible. A slight leak was 
observed in the condenser, but it was possible to read the 
potential before the loss due to this cause had time to become 
appreciable. It was verified that no effect was produced on 
the electrometer, either by the spark when no explosion took 
place, or by an explosion when the wire and cylinder were 
at the same potential. It follows, theretore, that the charge 
acquired by the wire is due to the action of the electric field 
in collecting the ions generated by the explosion. Under 
the action of the field, if the cylinder is kept at a positive 
potential, the positive ions are driven to the wire, the 
negative to the cylinder, and the wire acquires a positive 
charge. If the cylinder is kept at a negative potential, the 
reverse effect takes place; but the phenomenon might be 
further complicated owing to the generation of ions by 
collision, as the negative ions pass through the intense field 
of force around the wire. For this reason the cylinder was 
kept at a positive potential. 
It was found that the explosion would not take place if 
the pressure of the combined gases was much below 80 mm., 
and that explosions of the gases at a pressure above 100 mm. 
endangered the air-tight joints of the apparatus. The experi- 
ments were therefore confined to this range of pressures. 
The results are shown in the following table, where :— 
p =the pressure in mm. of the mixed gases before 
the explosion, 
p! =the fall of pressure in mm. resulting from the 
explosion and the absorption of ‘the water- 
vapour formed by it, 
p' =the pressure in mm. of the unexploded gas (viz. 
p'=p—-p), 
