684 
Proceedings of the Royal Soeiety 
error might have been less. Dr Lenz* has recently described an 
application of the telephone to the measurement of temperatures at 
a distance. Dr Lenz connects the observer with the distant station 
by means of a thermo-electric combination of wires, into the circuit 
of which he introduces a silent interrupter and a telephone. If 
the temperature of the junction at the distant station is not the 
same as that of the junction near the observer, a sound is heard in 
the telephones, which ceases when the observer has made the tem- 
perature of his junction the same as that of the distant one. This 
arrangement suggests a method of getting the true temperature of 
the air, by making the thermo-electric junction or junctions of as 
fine wire as possible, and exposing them freely to the air under 
shade at the distant station. In this way we might get as small a 
sensitive surface as it is possible to construct. 
On reconsidering the whole matter at this point, it soon became 
evident that it is hopeless to expect any thermometer of ordinary 
construction, however small the bulb, to give the true temperature 
of the air while it is exposed to radiation. When we consider 
what is taking place it is easy to see why this must be so. When 
radiant heat falls on the bulb of a thermometer, the heat is 
absorbed not only at the surface of the bulb, but all through the 
thickness of the glass, and at the surface of the mercury. The 
inside of the bulb therefore gets heated, and this heat must be 
conducted outwards through the glass before it can be carried 
away by the air. The consequence is, the inside of the bulb is 
hotter than the outside, and the thermometer while exposed to 
radiation must always read too high, however strong the current of 
air may be to which it is exposed. As the absorbed heat requires 
to be conducted to the outside, the inside of the thermometer must 
always be considerably hotter than the air, so long as the radiation 
temperature is higher than the temperature of the air. 
The natural sequence to these thoughts was — cover the bulb with 
something through which radiant heat cannot penetrate ; and as it 
would be necessary to use some substance having a small absorbing 
power for radiant heat, silver naturally suggested itself as the most 
suitable material for the purpose. I accordingly coated the bulb and 
part of the stem of the fine-bulbed thermometer with silver 
* Nature, voL xxx. p. 345. 
