560 Proceedings of the Royal Society of Edinburgh. [Sess. 
The following table gives some results obtained : — 
Radiant Efficiencies in per cent. 
Voltage. 
Carbon (1). 
Carbon (2). 
Tungsten (3). 
Tungsten (4). 
116 
0-12 
2“48 
184 
1-40 
0-72 
4-96 
5-98 
216 
1-97 
1-48 
7-42 
9-02 
250 
3-42 
2-42 
7-60 
9-39 
270 
3-95 
3-43 
8-06 
9-59 
Voltage. 
Osmium (5). 
30 
2-17 
45 
3-84 
55 
6-52 
Voltage. 
Tungsten (6). 
Tungsten 
(7)- 
| Tantalum (8). 
Tantalum (9). 
75 
1-61 
T69 
206 
3-00 
125 
6-61 
6*43 
6-66 
6-35 
146-5 
9-04 
8-83 
150 
7-70 
9-25 
All the lamps with the exception of the osmium are well-known makes 
much in use at present. The osmium lamp, which was invented by Auer 
von Welsbach, was the first metallic filament lamp to be a commercial 
success. It is not made now, and some trouble was experienced in getting 
one. The marked voltage of (1), (2), (3), and (4) was 250 volts; of (5), 
50 volts ; of (6) and (7), 130 volts ; and of (8) and (9), 125 volts. The lamps 
(1) and (2) were of different makes, the filament in (2) having a metallic 
lustre; (3) and (4) were the same make, (6) and (7) the same make, and 
(8) and (9) the same make. 
The figures in the table are probably accurate to 5 or 6 per cent. The 
difference in the behaviour of the 250- volt and 130- volt tungsten lamps 
I believe to be genuine and to be probably due to a different method of 
manufacture. In order to investigate this point thoroughly, however, it would 
be necessary to get special lamps made with one straight filament in each. 
If we calculate the radiant efficiencies of the different filaments at their 
marked voltage and take the mean, we obtain : — 
Radiant efficiency. 
Watts per candle. 
Carbon 
2-9 
3-5 
Osmium 
5-2 
1-5 
Tantalum . 
6-5 
1-7 
Tungsten 
7-5 
1-0 
