238 



PROCEEDINGS OF THE AMERICAN ACADEMY 



less for the same current in the inverted than in the upright arc, ancl 

 the same is probably true to a less degree with the silent arc, though 

 this is not so clearly shown in our observations, partly on account of 

 an exceptionally high value for the inverse electromotive force with 

 the current of 7.3 amperes and inverted arc. 



It will also be observed that the corresponding values of the con- 

 ductive resistance {bl) and the total equivalent resistance are both 

 somewhat less for the inverted arc. This differs from a result some- 

 what doubtfully recorded by Niaudet.* 



The points of intersection of the lines corresponding to the two 

 varieties of the inverted arc are given in the following table, as read 

 from the curves. They may also, of course, be obtained by solution 

 of the equations. 



TABLE XVIII. 



Comparing this with the table already given for the upright arc, 

 it appears that with the inverted arc the length at which the higher 

 inverse electromotive force sets in is somewhat greater. The corre- 

 sponding equivalent resistance is not greatly different in the two cases. 



Measurements were also made of the resistance of arcs into which 

 certain metallic salts had been introduced. The salt, in powder, was 

 placed on the lower (negative) carbon while this was quite hot, care 

 being taken entirely to cover the carbon point, and to keep the pow- 

 der from being jarred off on forming the arc. The reading of the 

 voltmeter for a given length of arc remained quite constant for a 

 short time after the arc was established, so that readings could readily 

 be obtained. 



Observations were made with arcs containing biborate of soda, sul- 

 phate of soda, and sulphate of potash. The arcs employed were 

 from ^ij to -^.y in. in length with the biborate of soda, from ^ to ^^ 

 with the sulphate of soda, and from ^ to ^| with the sulphate of 



* Machines Electriques a Courants Continus, p. 183. 



