Nov. 3, 1881 | 
NATURE 15 
thick. Its resistance was o'9 ohm. The three former | ments. Sheets of séee/ (palladium and platinum can also 
instruments were one after the other connected with a | be used) are rolled out until of a thickness of from ‘o1 to 
short-coil mirror galvanometer of sufficient delicacy. The | ‘002 of a millimetre is attained. Out of these sheets small 
same galvanometer was used in the bridge, the three re- 
sistances used with the strip being respectively ‘9, “4, and 
4 ohm, and the total current employed being a little over 
half a weber. 
three instruments and of the strip to heating by radiation 
to be respectively as 1, 4°1, 16°3, and 226°3. 
The actual bolometer embodies the principle of the | 
preliminary experiment with various additional refine- 
The result showed the sensitiveness of the | 
gratings are cut or punched, having the individual bars 
| about I millimetre wide and 1 centimetre long. Two 
systems of strips are arranged so that the current from a 
suitable battery divides itself, half passing through each, 
| the interposed galvanometer showing no deflexion when 
| the two currents are of equal strength. Fig. 1 shows the 
general arrangements of the gratings of strips. A rectan- 
| gular opening is cut in a disk of ebonite of 3 centimetres 
diameter. A second disk of the same size is clamped 
behind it, and between the two the gratings or systems 
of strips are fixed. That system which is to be exposed 
to radiation is placed in the centre of the rectangular 
opening at M. It consists of fifteen strips, eight of them 
being in front, and seven at a little distance behind. 
The second system is divided into two halves, N and N’, 
on each side of M, each half consisting of seven similar | 
strips, four in front, three behind. Every joint is soldered, 
and the resistance of the fourteen strips in N N’ is made 
up equal to that of the fifteen strips in M by the inter- 
position of a short wire in the circuit. 
in the diagram of Fig. 3. 
To protect the bolometer from air-currents, sudden 
changes of temperature, and from danger in handling, it 
is inclosed in a cylinder of ebonite lined with sheet 
copper. 
drawn partly in section to display the interior. At the 
anterior end of the tube is a revolving diaphragm with 
suitable apertures. Within, a number of cardboard dia- 
phragms or stops are placed, being retained in position 
by rings of ebonite tube between them. Behind these is 
the grating G of the bolometer fixed between the twe 
disks of ebonite A and B. At the back there is a layer oi 
solid non-conducting material, through which the con- 
ducting-wires pass to the two terminals. In the posterior 
end of the case are contained the resistance-wires by which 
to bring the two systems to equality ; this being advisable 
because, if they are unequal at the beginning of the 
experiment, though they can be balanced by taking pro- 
portionally unequal resistances in the other arms of the 
bridge, according to the well-known law, any general rise 
of temperature will produce a gveater increment of resist- 
ance in the system whose resistance is at first greater, 
producing a continuous “drift” 
needle. Fig. 3 shows the connections of the bolometer 
and the bridge. A battery of one or more Daniell’s cells, 
Z, C, provides a current the strength of whichis controlled 
at will by changing the resistances in a box of coils, R, | 
arranged as a shunt to the bridge-circuit. The working 
current is measured by a shunted galvanometer, g, and | 
M is placed in | 
one arm of the bridge, and N N’ in the other, as indicated | 
This is represented in Fig. 2, where the tube is | 
in the galvanometer | 
| the two systems of strips M and NN’ of the bolometer are 
connected to their respective places in the bridge by four 
insulated wires twisted together and covered with flannel. 
A modification of the usual formula enables the change of 
resistance of Mto be calculated from the currents observed 
in the galvanometer G. 
The results of the new instrument are somewhat 
startling. A sunbeam one square centimetre in section 
| will, according to Prof. Langley, warm one gramme of 
| water 1° C. in one minute. It would therefore raise’ a 
| sheet of water 1-500th of a millimetre thick, and 1-1oth 
“i? 
(on 
Ca 
of a square centimetre in area, 834° C. in one second, 
supposing all the heat to be retained. And as platinum 
| has a specific heat of only ‘032, the same sunbeam falling 
|on astrip of platinum of these dimensions should, on a 
similar supposition, raise it in one second to 2603° C., a 
| temperature sufficient to me/¢t it! This result is, how- 
| ever, prevented by the re-radiation which the strip almost 
| instantaneously exerts. 
An examination of the heating effect of rays from 
different portions of the spectrum of solar radiations was 
made, but under conditions different from those of the 
measurements made by Miiller, Herschel, and Tynda 
