654 ALEXANDER MACFARLANE AND P. M. PLAYFAIR ON THE 
This diminution of the difference of potential required to effect the dis- — 
charge must be due to a change at the surface of the wire ; for the air between 
the wires cannot be so greatly rarified by the heating of the wire as to produce 
the effect. 
Measurement of the Difference of Potential required to Pass a Spark through 
Air at Different Temperatures, the Pressure being Constant. 
To investigate this question we constructed the vessel represented in | 
fig. 2. A glass cylinder, ¢, fits into two brass plates, p, 
by means of grooves. The brass discs, d, which were 
those generally used in the experiments, were screwed — 
on, the one to a brass rod rising from the lower plate, — 
the other to a rod which moves inside a tube fixed to — 
the upper plate. The upper plate contains an orifice | 
for the purpose of allowing air to escape when the 
lower plate is heated, and also a hole for the insertion 
of a thermometer. The lower plate was put to earth, 
and the upper charged by being kept in contact. 
with a projecting conductor of the Holtz. The heat- 
Fig. 2. ing was effected by means of a powerful Bunsen 
burner placed below the lower plate, and was carried on- 
till the plate became red-hot. q 
















TasBLeE V.—Spark through Air at Different Temperatures, 6th June 1878. 
Electrodes, discs. Length of Spark, ‘9 centimetres. Pressure, atmo- 
spheric. 



HEATING. COOLING. 


Temperature ra i . 
in Degrees Deflection. Zero. me meas Deflection. Zero. ee ae 4 
Centigrade. : 
ts n. nv. nv —n. nN. nN. n—n. 

Eyre af | 40 463 423 
25 fh eBy ON E62 455 | 465 is 460 492 | 493 
30 453 493 


465 465 




