410 Messrs. J. Frith and C. Bodgers on the 



It was pointed out, however, by Prof. Ayrton at the 

 Ipswich meeting of the British Association, that although 

 there was a marked agreement between the values obtained 

 by these experimenters, they were not at all in accordance 

 with the conclusions drawn by himself from consideration 

 of the curves obtained by Mrs. Ayrton. These curves con- 

 nect the P.D. between the carbons with the current passing 

 through the arc, for various fixed lengths of arc, and from 

 them it is seen that for a given arc-length an increase of P.D. 

 is always accompanied by a decrease of current. From this 

 fact Prof. Ayrton concluded that if an attempt were made to 

 measure the resistance of the arc by altering the P.D. between 

 the carbons and finding the corresponding alteration of cur- 

 rent produced, the resistance found by taking this ratio must 

 be negative. 



This conclusion was strengthened by some experiments 

 made by Mr. Mather at Prof. Ayrton's suggestion. In one 

 of these experiments two points of equal potential were found 

 in a circuit consisting of an arc, a battery, and a resistance. 

 Another battery, consisting of a few cells, of known E.M.F. 

 and resistance was applied between these two equipotential 

 points and the current flowing through the battery was noted. 

 The resistances of the two parallel halves of the circuit, exclu- 

 ding the arc, were known, so that the current which, taking 

 the arc resistance as zero, should flow through this battery 

 could be calculated. Now the value of this calculated cur- 

 rent was found to be less than the observed value, no matter 

 in which direction the P.D. was applied, and this result was 

 also obtained when an alternating P.D. was used. Hence 

 the resistance of the arc was apparently less than zero. 



The other experiment consisted in running the arc at a 

 steady P.D. and current, suddenly altering the resistance in 

 circuit by a small amount, and noting the changes in the 

 ammeter and voltmeter-readings so produced. The new con- 

 ditions were maintained only long enough to allow of these 

 readings being taken. The arc was then brought back to its 

 former condition before taking another reading. It was found 

 that a change of P.D. in one direction was always accompanied 

 by a change of current in the opposite direction. The results 

 of both experiments were, however, only qualitative. 



All these experiments, together with the consideration of 

 the curves found by Mrs. Ayrton, lead to the conclusion that 

 the arc has a negative resistance, while former experimenters 

 had all obtained a positive resistance. 



It was in order to throw some light on this discrepancy 

 that we undertook a series of experiments to determine with 



