Current of Electricity from a hot Platinum Wire, etc. 495 



ions starting at distance a will of course be the ones most easily 

 deflected by the magnetic field. The same effect was also 

 observed in my experiments as will be seen later. 



In these experiments the platinum wire used was very pure, 

 its temperature coefficient between 0°C. and 100° C. being 0'0038. 

 The diameter of the wire was 0*2 mm. In the first apparatus 

 made the wire was fixed inside a glass tube in the form of a flat 

 grating, the plane of the grating being perpendicular to the axis 

 of the tube. Parallel to the grating and four millimetres distant 

 from it was an insulated aluminium plate connected to one pair 

 of quadrants of an electrometer. The diameter of the plate was 

 about 2'3 cms., and was six or seven mms. larger than the greatest 

 breadth of the grating. The grating was fixed to copper wire 

 leads and could thus be raised to any desired temperature by 

 means of an electric current. The platinum wire formed one arm 

 of a Wheatstone's bridge, the corresponding arm being a spiral of 

 thick german-silver wire (1'16 ohms resistance) immersed in a 

 bath of paraffin oil The two other arms of the bridge were high 

 resistances of the order of 1000 ohms, got from a post-office 

 box. The temperature of the wire was calculated from its 

 resistance in the ordinary way, the coefficient 8 for the wire 

 being I'd. 



The temperatures given in the table probably differ somewhat 

 from the actual temperature of the wire as only a short length of 

 wire (about 6 cms.) could be conveniently used. This, however, is 

 no great disadvantage, for the results obtained do not depend 

 upon an accurate knowledge of the temperature of the wire. 



The wire could be raised to various temperatures by altering 

 the resistance of a german-silver wire rheostat inserted in the 

 heating circuit. As the wire was to be charged to a high potential, 

 the heating battery, post-office box, etc., were insulated on paraffin 

 blocks. The wire was raised to a high potential by connecting 

 one end to one pole of a battery of small storage cells, the other 

 pole of which was connected to earth. The same end of the wire 

 was also always connected to a Weston voltmeter. The electro- 

 meter used was of the ordinary pattern giving a deflexion of 

 nearly 60 divisions for a volt. The needle was made of thin 

 aluminium having a platinum wire vane dipping in sulphuric 

 acid. The usual paraffin block electrometer keys were discarded 

 and replaced by mercury cups connected directly to the quadrants 

 of the electrometer. Great care was taken to see that no de- 

 flexion of the electrometer needle took place when the cold wire 

 was raised to a high potential. In making an observation of the 

 discharge, one pair of electrometer quadrants was at a definite 

 instant insulated, the other pair remaining earthed. The quad- 

 rants were now allowed to charge up for a certain number of 



