﻿Electricity in Flames containing Metallic Vapours. 867 



electrode, so that the uniformity of the field is not disturbed 

 by it. The presence of the carriers is detected by the current 

 through the galvanometer caused by the introduction of the 

 bead of salt into the flame. 



The earthed electrode is a massive brass plate, 6 cm. x 

 8 cm. x0'5 cm. ; to this is fastened a strip of brass pierced 

 by a vertical slot, in which runs a brass piece holding an 

 amber insulation : through the amber passes a stout wire 

 holding the horizontal strip of platinum foil, 4 mm- X 50 mm., 

 close to and parallel to the electrode. The brass piece can 

 be clamped by a screw at any given height, and its position 

 read off on a scale fastened to the slot. The other electrode 

 is a brass plate similar to the earthed electrode, but insulated 

 on amber ; it is joined up to a " Mercedes " influence machine, 

 through a reverser arranged so that either pole can be con- 

 nected to the electrode, the other to earth. Between the 

 electrodes, not touching either of them, burns a nonJuminous 

 flame of air and gas from a burner provided with a quartz 

 flame piece, giving a flat flame about 20 mm. wide and 6 mm. 

 thick, the plane of the flame being at right angles to that of 

 the electrodes. By means of a small electromagnet, the 

 current through which is controlled by a variable resistance, 

 a platinum loop containing the given salt can be brought 

 gradually into the flame ; the platinum wire is insulated 

 from the iron armature which supports it. The galvano- 

 meter, one pole of which is connected to the insulated 

 platinum strip, the other to earth, is of the D'Arsonval type, 

 with a resistance of 10,000 ohms ; it is usually shunted with 

 a resistance of 1900 ohms. 



The potential of the insulated electrode is kept constant by 

 a spark-gap connected in parallel, This potential is measured 

 on a rough electrometer ; the potential fall in the flame by 

 means of two parallel " probes ? of fine platinum wire placed 

 in the flame itself, and connected to a small aluminium leaf 

 electrometer, which is insulated, the outer case being con= 

 nected to one probe and the leaf to the other, The electro^ 

 meter is protected by an earthed net. For the smaller 

 potential falls used in the final and more accurate experu 

 ments on the velocities of the faster carriers, a bifilar 

 electrometer is used, the two threads being in this instrument 

 surrounded by an insulated cylinder inside the outer metal 

 case, which outer case is earthed, 



The probes are placed in the flame between the luminous 

 streak and the negative electrode ; it then makes practically 

 no difference in the potential fall whether the streak is 

 present or not, If the potential difference is measured in the 



3 h '2 



