402 Dr. Richardson: Effect of a Luminous Discharge 
necessary to give a name to a quantity which occurs so 
frequently. 
It seems advisable to explain at the outset that, despite 
numerous experiments, the true inwardness of the pheno- 
mena under investigation is still uncertain. However, a 
number of definite results have been obtained, of which a 
brief account might be of some interest. The present paper, 
then, is merely a short summary of the leading results, as 
the author considers it inexpedient to publish the full experi- 
mental details and the numbers obtained until he is better 
able to judge which experiments are of most importance. 
The general type of apparatus employed in these experi- 
ments consisted of a cylindrical discharge-tube furnished with 
two flat circular electrodes. Near one of the electrodes was 
inserted a spiral or loop of fine platinum wire which could be 
heated to any desired temperature. Most of the experiments 
consisted in measuring the emissibility imparted to this wire 
by passing the discharge between the two electrodes under 
various conditions. This arrangement of apparatus, which 
may be regarded as typical, was modified in connexion with 
special experiments in ways which naturally suggested them- 
selves. The temperature of the platinum wire was deter- 
mined from its resistance, To measure the emissibility the 
spiral was raised by means of cells to a potential which was 
generally 200 volts, but sometimes less; whilst the nearest 
electrode was connected with one pair of electrometer quad- 
rants, the other being earthed. In most of the experiments 
the temperature to which the wire was heated in order to 
drive off the imparted effect was about 650°.C. Except 
where otherwise stated, the vacuum-tube discharges used in 
these experiments were produced by a battery of storage- 
cells going up to 1000 volts. 
In every case cited in the previous paper, the wire which 
was revived by the luminous discharge formed one of the 
discharge electrodes. Experiments were therefore first made 
to see whether this was a necessary condition or whether a 
wire would not be revived by a discharge passing between 
two separate electrodes near it. The result of the experiment 
showed that a large effect was imparted to an insulated 
platinum wire 3 cms. distant by the discharge between two 
aluminium wire electrodes 1 cm. apart. 
Experiments were next made to test how the imparted 
effect varied with the distance of the testing wire from the 
discharge. As it will perhaps serve to make the method of 
experimenting clearer, a somewhat detailed description of the 
apparatus used for this purpose will be given. 
OO a ES 
