366 Prof. Pollock and Mr. Ranclaud on the 



incandescent carbon, leads to the following general description 

 of the distribution of the potential between the carbons : — 



1. A forward electromotive force at the cathode surface. 



2. A back electromotive force at the surface of the anode 



depending on the temperature of the crater. 



3. An anode fall of potential equal to the back electro- 



motive force at the anode. 



4. A cathode fall of potential, such that the result of its 



addition to 1 is a critical value. 



The description agrees with the facts as discovered by 

 Mr. Duddell. 



As the magnitudes of the above quantities are independent 

 of the current and arc length, the sum of 2, 3, and 4,. 

 minus 1, represents what has been called the back electro- 

 motive force cf the arc, in the case considered 38*3 volts. 



With the figures given, the velocities of the electrons are- 

 as follows : — 



At projection from anode, 2*5 x 10 8 centimetres per second. 



At projection from cathode, 1*5 X 10 8 centimetres pel- 

 second. 



At collision at end of cathode fair of potential 2*6 X 10 8 

 centimetres per second. 



At collision with anode surface, possibly 3' 6 X 10 s centi- 

 metres per second. 



The Physical Laboratory, 

 The University of Sydney, July 3rd, 1908. 



XXXIV. The Discharge of Electricity from Glowing Carbon. 

 By J. A. Pollock, D.Sc, Professor of Physics in the 

 University of Sydney, and A. B. B. Ranclaud, B.Sc* 



[Plate VI.] 



T 



1. Introduction. 



HE experiments, of which a description is here given, 

 were undertaken in connexion with an investigation of 

 the phenomena associated with the relighting of the carbon 

 arc. For the object in view, an arrangement was required 

 in which carbon rods should be situated somewhat as they 

 are in an arc-lamp, and in which the temperature of one of 

 the rods could be readily controlled. The plan adopted was- 

 as follows : — A cylinder of carbon, 4*5 centimetres long and 

 - 5 centimetre in diameter, was electrically heated, and a 



* Communicated by the Authors. Read before the Royal Society of 

 New South Wales. 



