Tests of Glow-Lamps. 



399 



keep the pressure throughout the night within one tenth per 

 cent, of the desired value, it was necessary to construct such 

 an instrument. 



The apparatus which we employed for this purpose resembled 

 generally the one described in a paper on " The Working 

 Efficiency of Secondary Cells" by Messrs. C. Gr. Lamb, E. W. 

 Smith, M. W. Woods, and one of the authors of the present 

 communication, see Journ. Inst. Elect. Eng. vol. xix. 1890. 



A variable resistance, consisting of four platinoid wires 

 winding on and off a brass roller, was placed in the main 

 circuit between the cells and the lamps. This resistance was 

 geared to a permanent magnet Gramme-motor, the electrical 

 connexions of which are shown in fig. 6. d ef was a battery 



Fig. 6. 



of five storage-cells to the centre e of which one brush of the 

 motor m was connected, the other brush being joined to the 

 middle cup b of a three-way mercury switch a b c. The two 

 outer cups a and c of this switch were connected with the two 

 ends d and / of the battery. 



It is clear from the figure that the motor revolved one way 

 or the other according as a and b or b and c were connected. 



A " set up " d'Arsonval galvanometer in series with a 

 resistance was placed across the two points in the main 

 circuit between which it was desired that the pressure should 

 remain constant. To the coil of this galvanometer was 

 attached a pointer ending in a platinum tip which worked 

 between two platinum contacts. The phosphor-bronze strip 

 by which the coil of the d'Arsonval was suspended was twisted 

 several times, so that when the pressure on the lamps was 

 correct the tip of the pointer rested midway between the 

 contacts. If, however, the pressure rose by one tenth of a 

 volt the pointer was deflected and made to touch one of the 



