Prof. E. Edlund's Investigation of the Electric Light, 109 



entire resistance of the current remains the same as before. If 

 these conditions are fulfilled, the electromotive force in the voltaic 

 arc is the same, whether its length be greater or less. The force 

 in question sends a current in an opposite direction to the prin- 

 cipal current, and therefore diminishes the total intensity. It 

 acts in diminishing the intensity in the same manner as an in- 

 terposed resistance. It is clear, on the other hand, that the 

 real resistance in the luminous arc must, under circumstances 

 otherwise similar, increase with its length. If, then, the resist- 

 ance in the arc be measured, it will be found that this consists 

 of two parts, one of which is independent of the length of the 

 arc, and the other increases with it. If the first resistance be 

 denoted by a, and the proper resistance in a luminous arc of the 

 length of a division of the scale by b v the entire resistance 

 measured in a luminous arc of this length is z=a + b x . If the 

 luminous arc has a length of two divisions and the mean resist- 

 ance in each is called b q , the entire resistance in this arc is =«-f 2& 2 . 

 In an arc of the length of three divisions of the scale it is 

 =za + 3b 3 , and so forth. The subsequent experiments show that 

 a has a very considerable value, and that b v b 2 , b 3) &c. are equally 

 great. 



The battery used was one of Bunsen's with carbon in nitric acid. 

 As rheostat, a wooden trough was used, coated on the inside with 

 asphalt. It was 440 millims. in length by 148 in breadth and the 

 same in depth, and was partly filled with solution of blue vitriol. 

 In the trough were two copper plates of the same breadth as it, 

 and reaching to the bottom. One of these was fixed ; the other 

 could be moved parallel to it, and the distance between the two 

 read off on the upper edge of the trough, which for this purpose 

 was graduated. The current came into the solution through 

 one plate and out through the other. In order to avoid the in- 

 fluence of polarization on the accuracy of the measurements, a 

 greater or a smaller part of the rheostat was always brought into 

 the conduction. The current could be measured by a tangent-com- 

 pass and be inverted by a commutator. As in these experiments 

 only currents of considerable strength were applicable, metal 

 wires could not be used as rheostat, because they would have 

 been very strongly heated, and thereby have had their resistance 

 altered. 



The poles between which the luminous arc was formed were 

 placed in Foucault's apparatus for producing constant electric 

 light. In most experiments, however, the apparatus was only 

 used as a pole-holder, and the definite distance between the 

 points produced by moving one of the pole-points by the hand*. 



* Professor Stenberg, of the Carolinian Medico- Chirurgical Institute, 

 who took part in all these experiments, determined the length of the lu- 



