TRANSACTIONS OF SECTION A. 401 
6. Huperiments in Bolometry. By Professor Smvanus P. Tuompson. 
Hitherto the bolometers devised by Langley and others have been constructed 
of thin metal films blackened with lamp-black or platinum-black. Abstract con- 
siderations show, however, that to have a maximum degree of sensitiveness the 
conductor employed should be one in which the change of electric resistance per 
degree of temperature is the greatest; it should also have as small a thermal 
capacity as possible, and its exposed surface should be its only conducting part. 
Theoretically, therefore, a carbon-film in some high-conducting form should be the 
best. Pending the preparation of special carbon-films the author had made some 
experiments on the strips and filaments of carbon which are to be found in incan- 
descent lamps. The resistance of these, when carefully measured on a Kirchhoft’s 
bridge, was found to be considerably less when exposed to radiation than when 
kept in the dark, the change of resistance in some cases amounting to 3 per cent. 
of the total resistance. The cylindrical filaments and rectangular strips used in 
most lamps are not, however, well adapted to show the bolometric effect, as their 
exposed surface is small as compared with the area of the cross-section of the 
conductor. 
These observations suggest the following points :— 
(1) That measurements made for specifying the resistances of such lamps 
should be conducted in the dark. 
(2) That a carbon-film, or even a carbon-lamp, may serve as the receiving part 
of a photophone. 
(3) That probably the effects obtained by Dr. Bornstein with metals might be 
re-observed if the metal films were very thin. 
(4) That special films of good-conducting carbon should be applied in standard 
bolometers. 
(5) That a true standard photometer might be constructed on the bolometric 
principle, having the sensitive conducting film covered with pigmentum nigrum 
from the human eye. Such an instrument ought to be sensitive to precisely the 
same rays as the eye itself, and in identically relative proportions for ditferent 
rays. 
7. On the Equations of Motion and the Boundary Conditions for Viscous 
Fluids. By Professor Osporye Reyyoups, F.R.S. 
8. Suggestions for facilitating the use of a delicate Balance. 
By Professor Lorp Rayuetcu, F.R.S. 
In some experiments with which I have lately been occupied a coil of insulated 
wire, traversed by an electric current, was suspended in the balance, and it was a 
matter of necessity to be able quickly to check the oscillation of the beam, so as to 
bring the coil into a standard position corresponding to the zero of the pointer. A 
very simple addition to the apparatus allowed this to be done. The current from 
a Leclanché cell is led into an auxiliary coil of wire, coaxal with the other, and is 
controlled by a key. When the contact is made, a vertical force acts upon the 
suspended coil, but ceases as soon as the contact is broken. After a little practice 
the beam may be brought to rest at zero at the first or second application of the 
retarding force. 
This control over the oscillations has been found so convenient that I have 
applied a similar contrivance in the case of ordinary weighings, and my object in 
the present note is to induce chemists and others experienced in such operations 
to give it a trial. Two magnets of steel wire, three or four inches long, are 
attached vertically to the scale-pans, and underneath one of them is fixed a coil of 
insulated wire of perhaps 50 or 100 turns, and of 4 or 5 inches in diameter. The 
best place for the coil is immediately underneath the bottom of the balance-case. 
It is then pretty near the lower pole of the magnet, and is yet out of the way. 
The circuit is completed through a Leclanché ce!l and a common spring contact- 
1883. DD 
