MR. 0. V. BOYS ON THE RADIO-MICROMETER. 
181 
But to do this the ordinary methods must be discarded, and special means and special 
tools devised, when the difficidty becomes greatly reduced. Though full instructions 
would be interesting to the few who would ever probably care to make circuits of this 
extreme fineness, I do not think a detailed account of the manipulation would be suitable 
for insertion in this paper. I may, however, mention two causes, ordinarily of no great 
moment, which become, under the peculiar circumstances, of the first importance. 
One is the apparently instantaneous conduction of heat, and the other the surface 
tension of melted solder, which, unless provided against, will produce troublesome and 
unexpected results. There is no need to do more than mention the fusibility of the 
bismuth in the presence of the melted solder. 
If the length of the bars be supposed increased in the ratio of 1 : K will become 
nK, and C will become nC ; thus, since E oc Ij E will become n times as small. 
But here again less heat will reach the cool junction with the larger bars, both by 
conduction and by the Peltier action of the current. Thus, the warm junction will 
become warmer, and the cool junction colder: now, should the temperature difterence 
become also n times as great, the actual sensibility of the circuit would remain 
unchanged. If no heat were radiated from the bars, then the temperature difference 
would be proportional to n, and the actual sensibility independent of n ; but, on 
account of the radiation which must occur, the temperature difference would vary in a 
less ratio than L :n, and therefore the bars could not be too short until the cold junc¬ 
tion became sufficiently near the hot junction for it to be impossible to prevent the 
radiant heat from falh'ng on it also. 
On the other hand, on account of the increased flow of heat with the shorter bars, 
the cool junction would be made warmer, and the whole junction would therefore 
become warmer, and so there would be an increased loss by radiation from the warm 
junction. On this account the temperature difference would be less, 
I may mention here that,- in the narrow pattern of instrument, I have found it 
advantageous to make a special heat-receiving surface of the thinnest copper, of the 
size and shape suited to the purpose for which the instrument is made, and to keep 
the whole of the bars screened from the radiant heat altogether. This an adaptation 
to the radio-micrometer of the copper-faced thermopile, which Lord Bosse has found, 
and which is, obviously, so far preferable to the ordinary construction. 
Though it is impossible to find by calculation the exact relative value of the three 
sources of equalisation of temperature in the circuit—namely, conduction of heat, 
Peltier effect, and radiation—it will be some guide to find, as far as data will allow, 
what their values are. It is most coDvenient to express them all by giving the time 
that would elapse before all the heat which is transferred to the cold junction by 
either of the first two actions, or which escapes in consequence of the third, would be 
sufficient to raise the pair of bars to the temperature of the warm junction, or, in the 
case of radiation, to raise it about half as much. 
