1895.] Temperature of the Crater of an Electric Arc Light. 175 



Radio-Micrometer. 



o 



A Boys's radio-micrometer, with its aperture reduced to about 

 2 mm. diameter, was so placed on the pier in the laboratory that the 

 image of the crater fell on its small aperture. The instrument thus 

 gave deflections proportional to the radiation coming from the crater. 

 The current was supplied from a battery of accumulators, giving an 

 E.M.F. of 110 volts. Suitable resistances of platinoid wire were put 

 in the circuit, so that the current could be varied from 40 to 10 

 amperes. An ammeter was also in circuit, and the poles of the arc 

 were connected to a voltmeter. 



The gas used was nitrogen, and the pressure was got by connecting 

 the box by a copper pipe with the valve of a 20-foot steel cylinder 

 filled with the gas at a pressure of 120 atmospheres. A "["-joint on 

 the copper pipe was connected with a Bourdon pressure-gauge, which 

 showed the pressure in the box at any moment. 



The method of experimenting was first to start the arc with the 

 pressure in the box at that of the atmosphere. The image of the 

 brightest part of the crater was thrown on the aperture of the radio- 

 micrometer, and a series of observations taken of the deflections of 

 the instrument. The pressure was then gradually increased and the 

 maximum deflections observed. As the pressure rises the resistance 

 of the arc increases, and, in order to keep the same current flowing, 

 the resistance in the circuit was reduced. It soon became evident 

 that, even with moderate pressures of about 5 atmospheres, the tem- 

 perature of the crater had fallen. This was not only shown by the 

 induction in the deflections of the radio-micrometer, but also by the 

 fall in brilliancy of the image of the crater to the eye. The pressure 

 was then increased to about 20 atmospheres, and the brilliancy of the 

 crater fell to a dull red colour. These experiments were repeated 

 several times and always with the same results. 



I then tried the effect of reducing the pressure in the box by 

 means of an air-pump, but as some of the glands in the box were only 

 intended for an internal pressure, I found it impossible to get a 

 good vacuum ; yet by keeping the pump at work, and thus getting a 



YOL. LVIII. 



