40 



REPORT — 1900. 



the bridge-wii-e in order to allow for any minute residual difference o£ 

 temperature between the cube and the disc when the apparatus was 

 screened from radiation. The time of exposure was recorded on an 

 electric chronograph by the dropping of the screen on a suitable key. 

 The sliding contact was then shifted to successive points on the bridge- 

 wire, and the moment of balance at each point was observed and recorded 

 on the chronograph. These observations were continued for about five 

 minutes, or as long as the rise continued sufficiently rapid. Occasional 

 observations were then made to determine the final steady difference of 

 temperature B° during the next fifteen minutes, after which the tempera- 

 ture remained steady. 



The following is a sample of observations taken with the copper-cube : 



Datr, Apn7 4, 1900. Ohset-vev, Miss W. E. Walkee. 



Temperature of Clark cell, 19^0° C. ; bridge-wire, 20'2°C. ; resistance 

 per cm. of B.W., ■001091 ohm ; P.D. per cm., ■7795 microvolt ; diameter 

 of disc, 1-40 cm. ; diameter of aperture in cube, I'OO cm. ; distance of 

 lamp from aperture, 60-0 cms. ; volts on lamp, 77^2 ; mass of copper disc, 

 0-8320 gramme ; J?«s/A=^4206. 



SoJufinn of tJu' Fqnafions. — Assuming the elementary theory of the 

 method as given by equation (2), the simplest method of procedure is to 

 take the value of the ratio r/q as given by the final .steady difference of 

 temperature 6°= 1^090, and to calculate the values of q from the inter- 

 mediate observations of t and by substituting the observed value of 

 rjq in equation (2). AVe thus obtain 



qt=2-3026 logio^°/(^°-^) (3) 



The value of r is then found by the relation ?-=q'0, and the intensity of 

 radiation I by multiplying r by the constant factor J'ins/A=-4206. The 

 values thus obtained are given in the columns headed q, r, I. They 

 invariably exhibited a progressive increase with the time. The value of q 

 could also be found by eliminating the ratio o-/q between any two 

 observations, and solving the equation by trial for q. Taking the observa- 

 tions (1) and (2) at 39^8 and 59^S cms. above given, we thus obtain 

 ^^=•00967, whence r=-01103, and I=-00464, which illustrate the same 

 tendency, being smaller than the results obtained by assuming tlie ratio of 

 r to q from the final steady temperature. The focus-lamp in this 

 experiment was set to shine through an aperture of nearly the same size 

 as the incandescent grid, but this was found to be unsatisfactory, as the 

 field of illumination was not sufficiently uniform for the bolometric 

 receivers. This considei'ation, among others, ultimately necessitated the 

 abandonment of the aperture method of limiting the radiation received by 

 the disc. 



Effects of Lag. — It was clear from the results above quoted, and from 

 a number of others obtained with the same apparatus with different discs 



