116 Prof. E. Edlund's Investigation of the Electric Light. 



rature in a piece of platinum wire of the length of 0*4 millim. to 

 more than 2000 degrees, if this could happen without the wire 

 melting or being changed into gas. It is thus evident that the 

 resistance which, from what precedes, is actually present in the 

 luminous arc is sufficient to explain the production of the high 

 temperature which is observed there. 



Sixth Series of Experiments. 



9. When carbon points were used in the formation of the lu- 

 minous arc, the intensity was not constant during a long time, 

 but changed after short intervals. This was due to the circum- 

 stance that the luminous arc was not steady, but passed from 

 one point to another on the carbon points; and this displacement 

 was often accompanied by a crackling sound. The magnetic 

 needle hence began to move, by which the reading off was rendered 

 more difficult. Hence it not unfrequently happened that for a 

 single observation a pretty long time was needed, because the 

 reading-off had to be delayed until the needle was at rest. The 

 experiment was made with several kinds of carbon ; yet all pre- 

 sented this inconvenience, which only entirely disappeared when 

 a couple of thick copper wires were used instead of charcoal 

 points. The intensity was now constant, and only diminished 

 as the distance between the wires was increased. The subse- 

 quent determinations furnish the result of the experiments made 

 with the copper wires. 



Experiment X. The battery consisted of seventy-six Bun- 

 sen's elements. When 4 inches of the rheostat was interposed, 

 the tangent-compass indicated 54° 15' in the mean; and when 

 13 inches was interposed, 40° 50'. From this are calculated 

 M = 10-82 and E = 20*58. When 0'7 inch of the rheostat was 

 thereupon interposed in the circuit and formed a luminous arc 

 of six divisions, the magnetic needle remained at 53°. With this 

 intensity the following results were obtained : — 



Length of the arc = 6 divisions. Resistance =4*2 



=4 „ „ -40 



„ —2 „ „ =3-8 



„ =1 „ „ =3*6 



From this are obtained a = 3*4 and & = 0'2. 

 And by simple calculation we have 



r> 3-4 E 



1) = =4*4o. 



1572 



Experiment XI. Twenty of the elements were removed from 

 the. battery, so that only fifty-six were at work. W T hen 4 inches 

 of the rheostat were interposed in the circuit, the intensity was 



