36 Mr. A. H. Pfund: A Study 
the circuit immediately after extinguishing the light. If no 
changes had occurred, the galvanometer would indicate a 
deflexion corresponding to the former resistance of a cell in 
darkness; but, if a change had occurred, the galvanometer 
would indicate a larger deflexion, and would gradually settle 
back to its original position. Asa preliminary experiment 
bore out these conjectures so admirably, an exact determina- 
tion was undertaken. 
A new cell was made and, upon closed circuit, was exposed 
to light for 30 seconds. The galvanometer-needle, upon 
extinction of the light, moved gradually towards its original 
position and, at definite time-intervals, readings of the 
galvanometer were taken. Then, with the cirewit open, the 
cell was again exposed for 30 seconds, and, at the same 
intervals as before (after turning off the light), the cireuit was 
closed for an instant, just long enough for the galvanometer 
needle to come to rest so as to obtain a reading. An inspec- 
tion of the following Table II. and fig. 4 shows that, at 
corresponding intervals of time, the cell had the same 
resistance whether a current had been flowing during the 
time of exposure or not. 







TABLE II. 
3 4 
Circuit closed during the pia eee: ek 
exposure of 30 seconds. 30 nae ies: 
seconds. 
Tie Galvanometer Galvanometer 
; deflexions. deflexions. 
min. © secs. mms. mms. 
0 205 0 
| 30)* 4) 390 0 
1 00 240 239 
| Ls 236 0 
2 OD) faa 230 229 
30 227 (0) 
3 00 _ 224 223 
(tio) 221 0) 
4 00 219 218 
5 00 915 215 
6 00 212 212 
| 7 00 209 0 
| 8 00 207 207 



| 
On account of the first rapid drop in the curve, and also 
due to the comparatively slow motion of the galvanometer- 
needle, it was not possible to obtain readings on the steeper 
parts of the curve. However, the excellent agreement 


