. Instruments and methods used in radiometry, III - The photoelectric cell and other selective radiometers . s. Jour., 39. p. 433; 1914.*^ Hulburt, Astrophys. Jour., 42, p. 210; 1915. Coblentz] Selective Radiometers 523 Hulburts grounding key was a pointed brass rod touching asmall brass plate, made from the same piece of brass. It gave notrouble with contact difference of potential. Sulphur was usedfor supporting the wires leading to the electrometer.^^ Nichols and Merritt ^^ have described a method of using thephotoelectric cell, whereby the deflection rather than the rate ofchange of deflec
. Instruments and methods used in radiometry, III - The photoelectric cell and other selective radiometers . s. Jour., 39. p. 433; 1914.*^ Hulburt, Astrophys. Jour., 42, p. 210; 1915. Coblentz] Selective Radiometers 523 Hulburts grounding key was a pointed brass rod touching asmall brass plate, made from the same piece of brass. It gave notrouble with contact difference of potential. Sulphur was usedfor supporting the wires leading to the electrometer.^^ Nichols and Merritt ^^ have described a method of using thephotoelectric cell, whereby the deflection rather than the rate ofchange of deflection is read. The electrical connections are shownin A, Fig. 7. The cathode of the photoelectric cell is connectedto one pair 3f quadrants of an electrometer and also through aresistance, R (capillary tube of absolute alcohol with adjustablewire immerson), to earth. The anode (platinum wire) is con-nected to tte plus side of a iio-volt battery. To balance the** dark cinrett of the cell they connected the other pair of quad-rants througi a variable resistance to the battery and to earth. /7 - MOV g ^ r©. Fig. 7.—Aiangement of apparatus used by: A=Nichols and Merritt; B=Richtmyer By adjustQg 5 and T the two pairs of quadrants were broughtto the saie potential when the cell was unilluminated. Thedeflectior of the electrometer upon illuminating the cell was ameasure f the intensity of the incident light. Richttyer ^^ has described a null method of making measure-ments Vith photoelectric cells and an electrometer. This evadesthe quedon of proportionalty of response (photoelectric current)in the ell. The arrangement of the apparatus is shown in B^Fig. 7. This requires two photoelectric cells, C^ and C^ and twosources)f potential, e^ and ^2, which can be varied from o to 120volts. The minus terminal of e^ and the plus terminal of e^ areconnec^d in sort of a Wheatstone Bridge to the photoelectric *2 For fufler information concerning keys for connecting and disconnecting the photoele
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