1887.] 



On Photometry of the Glow Lamp. 



251 



what sluggish. Mr. Yarley supplied us with one of his carbonised 

 cloth resistances, which consists essentially of a series of square pieces 

 of carbonised cloth more or less in contact. The figure represents 

 the one we had made for us. The carbonised cloth is represented 

 by C, fig. 3, which fills the whole length from A to D when loosely 

 packed. At B is a plate to which T 3 is attached, and which can be 

 separated more or less from a fixed metal plate to which T 1 is con- 

 nected by the arm E, which is moved by the screw S x . At A is an 

 insulated block carrying another plate to which T 2 is attached, and 

 A can be carried backwards or forwards by means of the screw S 2 . 

 For some purposes the main current can be brought in at T 3 , and 

 leads be taken from T 2 and T x , thus forming part of a Wheatstone 

 bridge. When only one resistance has to be inserted, T T and T s , 

 T l and T 2 , or 1\ and T 3 may be used for connecting on to the leads 

 with one pole of the battery and the ampere-meter. It was in this 

 way the resistance was used in the case in point. The lamp L u by 

 the use of this could be raised from black heat to bright white, and 

 a small turn of the screw altered the resistance very considerably. 

 We used two sets of sectors ; one pair enabled us to use an aperture 

 from 135° to 90°, and the other pair from 90° to 0°. The light from 

 L-l was diminished by the first pair of sectors being placed on A 

 (fig. 2), the current cc potential being noted. The resistance of the 

 current passing through L n was then altered till the illumination of the 

 two shadows on the screen appeared equal, the screw S (fig. 3) being 

 turned backwards and forwards, first one shadow and then the other 

 being made to appear too dark. By diminishing the oscillations 

 the neutral point can be very readily arrived at, even though the 

 colours of the lights may be very different (see Bakerian Lecture, 

 1886, " Colour Photometry," by the authors). The readings of V n 

 and A-q were then read and noted. The apertures of the sectors 

 were altered, and the same operations gone through. From observa- 

 tions thus made the curves were constructed, enabling the theory 

 propounded to be tested. 



The grease-spot plan of photometry was arranged in a somewhat 

 similar manner, L n being on the opposite side of the screen SS, and 

 the rod being abolished. In this method the room has to be dark 

 so as to admit of no reflection. At first we were not prepared for 

 any great exactitude with it, but finally we came to the conclusion 

 that it was very reliable, a conclusion that Mr. W. H. Preece* came 

 to when he constructed his photometric arrangements. 



Having described the arrangements for taking the measurements, 

 it remains to give the conclusions at which we arrived after making a 

 large number of experiments. 



* ' Koy. Soc. Proc.,' vol. 36, 1834, p. 270. 



VOL. XLIII. 



u 



