1895.] The Electrical Measurement of Starlight. 



147 



As the electromotive force produced by the light of the sun falling 

 directly on the cell is probably too large, it will be desirable to 

 diminish its intensity by taking it through a small measured aperture 

 and placing the cell at a known distance behind. 



The Electrometer employed. 



The instrument employed for measuring the electromotive forces 

 generated by the light of different stars is a quadrant electrometer 

 differing from the forms in ordinary use in having its quadrants 

 made of aluminium, two of these being supported on brass pillars 

 connected with the case of the electrometer and always earthed, while 

 the other two are supported on pillars of melted quartz. The quad- 

 rant box is about 2 cm. high and 5 in diameter; the needle is of 

 thin aluminium foil, cut into the peculiar shape figured in Clerk 

 Maxwell's ' Electricity and Magnetism,' and is suspended by a quartz 

 fibre about 9 cm. long. The needle and quadrants are surrounded 

 by a thick metal case, and the instrument is both air-tight and induc- 

 tion-tight. 



It had been intended to use with the electrometer an air condenser 

 consisting chiefly of two gilt brass plates, each about 15 cm. in 

 diameter, to multiply the potentials indicated by the electrometer ; 

 but at present there are difficulties in the way of its employment, 

 and the measures made on this occasion were made by the electro- 

 meter alone. 



Both instruments were constructed with the aid of the Government 

 Grant administered by a Committee of the Royal Society and were 

 made by Mr. Paul of Hatton Garden. 



The Telescope. 



This was Mr. Wilson's 2-feet reflector, which was at first used as 

 a Cassegrain instrument, and subsequently as a Newtonian, the cell- 

 carrier being in each case fixed to the telescope in place of the eye- 

 piece. 



Tiie Cell-Oarrier. 

 Fig. 3 represents the cell-carrier in plan. 



FIG. 3. 



