Prof. E. Edlund's Investigation of the Electric Light. 113 



When 0*6 inch of the rheostat was interposed in the circuit and 

 the carbon points pressed hard against each other, so that the 

 contact was complete, the deflection of the compass amounted to 

 60° 35 y . When 13*5 units of the rheostat were interposed, the 

 deflection was 42° 55'. From this we can calculate that the total 

 resistance external to the rheostat, or M, amounted to 13*62 

 units of resistance. Taking as unity the electromotive force 

 which for the resistance 1 produces a deflection of 45° on the tan- 

 gent-compass, the electromotive force of the battery or E = 25*22. 



When 0*6 inch of the rheostat was interposed in the circuit, 

 and the luminous arc was 4 divisions of the scale in length, the 

 tangent-compass remained at 48°*5. When 9 inches was there- 

 fore interposed and the carbon points pressed to contact, the 

 same number of degrees was obtained. The resistance in a lumi- 

 nous arc of the length of four divisions was thus =8*4. 



Retaining the same intensity of current, the following results 

 were obtained for shorter arcs : — 



Length of the arc =3 divisions. Resistance == 7*8 



}> a —" )> )i == ' '2 



From which is obtained a = 6'0 and b = 06. 



If, therefore, D be calculated in the same manner as before, 

 by the equation 



E E-D 



13-62-f9*0=22*62~22*62-6*0:=16*62' 

 we obtain 



D = 0-2652 xE= 6-69. 



Experiment V. Twenty-four of the elements were detached 

 from the battery, so that it only consisted of fifty-five. When 

 0*6 inch of the rheostat was interposed in the circuit and the 

 points were in contact, the needle of the tangent-compass re- 

 mained at 63° 32'; and when 13*5 inches was interposed, at 

 38° 59'. From this it may be calculated that the entire resist- 

 ance outside the rheostat, or M, = 8*104, and E'= 17*483. 



When 0*6 inch of the rheostat was interposed in the circuit, 

 and the length of the luminous arc was 2 divisions, the tangent- 

 compass indicated 43° 20'; and after 10*2 inches had been 

 brought into the circuit and the carbon points were in contact, 

 the same number of degrees was obtained. From this we get 



Length of the arc = 2 divisions. Resistance = 9 6. 



We have now, therefore, a = 7'2 and £ = 1-2. Calculating D 

 in the same manner as before, we obtain 



D = 0*3934xE' = 6*877. 

 Phil. Mag. S. 4. Vol. 35. No. 235. Feb. 1868. I 



