RADIATION IN ABSOLUTE MEASURE. 
441 
Extract from Note-book, date June 22 , 1886. (Pressure to be about 4 ^ mm. ; 
actual pressure - 4 \mm.) 
Observed. 
Calculated from observations. 
Current 
McLeod gauge. 
(1) 
(2) 
(4) 
(6) 
TT 
gal- 
W ater- 
Current 
Resist- 
(3) 
Tempe- 
(5) 
C J K/Js. 
Hour. 
vano- 
jacket. 
(abso- 
ance of 
r lr 15 . 
rat lire 
C-R (absolute). 
meter. 
P. 
V. 
lute). 
wire. 
of wire. 
Emission. 
Ohms. 
A.M. 
O 
n 
Im* 
938-0 
15-4 
9-6 
35 
1 
889-5 
bJD 
. . 
•0174 
2-590 
1-068 
40 
•7847 xio 6 
27 99 x 10-* 
2 
742-5 
• rH 
9-6 
35 
•0348 
3-118 
1-284 
118 
3-773 „ 
134-6 
3 
567-7 
•0522 
4-105 
1-691 
282 
11 16 „ 
398-2 „ 
4 
472-0 
go’ 
9-8 
35 
•0696 
4-952 
2-041 
430 
24-52 
874-6 
5 
424-2 
£ CD 
•0870 
5-521 
2-275 
530 
41-79 
1490 „ , 
6 
397-9 
o 
10 
, # 
•1044 
5-889 
2-428 
592 
64-19 
2290 
7 
368-3 
CD ^ 
, , 
•1218 
6-371 
2-629 
677 
94-3 
3362 
8 
344-0 
a 
Pressu 
reflow 
•1392 
6-826 
2-817 
752 
105-2 
3752 
9 
325-0 
EH 
10 
10 
•1566 
7231 
2-980 
820 
177-2 
6319 
(1) Current calculated by knowing galvanometer constant, which w r as found by electrolysis, (p. 432.) 
(2) Resistance of wire from observation of position of slider. (See p. 431 and fig. 1.) 
(3) r/r 15 , ratio of resistance of wire to its resistance at 15° C. 
(4) Temperature found from (3). (See pp. 435, 436.) 
(5) C 2 R, electric energy (absolute). 
(6) i3 (5) divided by J (Joule’s equivalent) and by s, the surface of the wire. 
An examination of the emission curves shows very clearly the part taken by the air 
in carrying away heat from the wire; and, in the process of experimenting, the 
diminution in the amount of this phenomenon produced by diminution of pressure is 
very striking. It will be seen from the curves that but little diminution of carrying 
power is produced by reducing from 50 mm. to 5 mm., or even to 2-| mm., and it is very 
remarkable (p. 445) how small is the effect produced by reducing from 760 mm. down to 
50 mm. On reducing to 1 mm. or to \ mm., however, the effect produced by the 
smallest alterations of pressure is very striking; so much so that it would be easy to 
show it even as a popular or lecture-room experiment. Suppose a steady current 
flowing through the radiation wire, fig. 1 , which is in a vacuum of \ mm.; and 
suppose that a balance has been obtained on the Wheatstone bridge, the testing- 
galvanometer giving the zero or nil indication. Let the mercury of the Sprengel 
* Im., immeasurably small. Taken at commencement of each series, partly to ascertain that electric 
connections are right. 
t The pressure was increasing slightly; the pump was worked and pressure diminished too much. 
The gas coming from wire puts this to rights after a little time, or else a minute quantity of air is 
admitted from one of the air-traps. It has been impossible for me to describe all the minutim of working- 
in this paper. 
MDCCCLXXXVII.-A. 3 L 
