560 



REPORT — 1881. 



Two galvanometers were used simultaneously, one {called the potential gahano- 

 ineter) for measuring- the difference of potentials between the two terminals of the 

 lamp, the other (called the current galvanometer) for measuring the whole 

 strength of the current through the lamp. 



The potential galvanometer had for its coil several thousand metres of No. 50 

 (B.W.G.) silk-covered wire (of which the copper w.-ighs about ^-^ gramme per 

 metre, and therefore has resistance of about 3 ohms per metre). Its electrodes 

 ■were applied direct on the platinum terminals of the lamp. 



The current galvanometer had for its coil a single circle of about 10 centimetres 

 diameter, of thick wire placed in the direct circuit of the lamp, by means of elec- 

 trodes kept close together to a sufficient distance from the galvanometer to ensure 

 no sensible action on the needle except from the circle itself. The directive force 

 on the needle wliich was produced by a large semi-circular horse-shoe magnet of 

 small sectional area was about 2i c.g.s., or 15 times the earth's horizontal magnetic 

 force in London. This arrangement would have been better for the potential 

 galvanometer also than the plan actually used for it, which need not be described 

 here. The scale of each galvanometer was graduated according to the natural 

 tangent of the angle of detiectioii, so that the strength of the current was simply 

 proportional to the number read on the scale in each case. 



Three lamps were used, Nos. II. and III. of a larger size than No. T. The ex- 

 periment was continued with higher and higher potentials on each lamp till its 

 carbon broke. 



The illuminating power was measured in the .simplest and easiest way (which 

 is also the most accurate and trustworthy), by letting the standard light and the 

 lamp to be measured shed tlieir lights nearly in the same direction on a white 

 ground (a piece of white paper was used) ; and comparing the shadows of a suit- 

 able object (a pencil was used) ; and varying the distance of the standard light 

 from the white ground till the illuminations of the two shadows were judged 

 equal. The standard used was a regulation ' standard candle,' burning 120 grains 

 of wax in the hour. The burning was not actually tested by weighing; but it was 

 no doubt very nearly right ; nearly enough for our purpose, which was an approxi- 

 mate determination of the illuminating powers of each lamp through a wide range 

 of electric power applied to it. The following results were obtained : — 



Lamp No. I. 



