1890.] between Electrodes at different Temperatures. 



123 



against it by a magnet, it is found that this third or insulated pole 

 is brought down almost to the potential of the negative carbon of the 

 arc, and that a galvanometer connected between the third insulated 

 carbon and the negative carbon of the arc indicates no current, but 

 that if joined up between the positive carbon and the middle carbon 

 a strong current of about an ampere or so is found to be passing. If 

 an electric bell or an incandescent lamp is joined up between the third 

 carbon and the negative carbon of the arc, they do not work ; but 

 if the bell or the lamp is joined between the positive carbon of the 

 arc and the third carbon, they are set in action by a strong current 

 passing through them. These effects are produced, although the 

 third carbon (which is best held at right angles to the other two 

 forming the arc) is half or three quarters of an inch away from the 

 positive and negative carbon, the sole condition being that the flame 

 of the arc must touch or be projected by a magnet so as to touch this 

 third carbon. We have, therefore, similar phenomena in the case of 

 the arc and incandescence lamps. 



(,20.) When the electric arc is being projected against the third 

 carbon, and has brought it down to the same potential, a galvano- 

 meter joined in between the two carbons shows no current ; but this 

 space between the negative carbon of the arc and the third carbon 

 possesses a unilateral conductivity, and will pass the current from a 

 small battery of secondary cells one way, but not the other. The 

 secondary battery when joined in series with the galvanometer 

 sends a current, if its negative pole is in connexion with the nega- 

 tive carbon of the arc, and its positive pole, through the galvano- 

 meter, with the third carbon ; but if the secondary battery is reversed 

 in position it sends no current. Negative electricity can pass along 

 the flame-like projection of the arc from the hot negative carbon to 

 the cooler third carbon, but not in an opposite direction. 



(21.) If the arc is projected by means of a magnet for a long time 

 against the third insulated carbon, it craters it out in the same fashion 

 as the crater of the positive carbon, and the tip of this third carbon, 

 where it has received the flame-like blast of the arc, is converted into 

 graphite. 



The same effects are observed if an iron rod is used as a third pole, 

 and in this case the end is converted into steel, and rendered so hard 

 as to be scarcely touched by the file when it has been quenched in 

 water. 



In seeking for an hypothesis to connect together these observed 

 facts, the one which suggests itself as most in accordance with the 

 facts is as follows : — 



In the case of a carbon incandescence lamp when at vivid incan- 

 descence, carbon particles are being projected from all parts of the 

 filament, but chiefly from the negative half of the loop. These carbon 



