On the Rotation of the Electrical Light. 463 



and the object, that is the arm of the balance, coincide. The in- 

 strument may readily be adjusted by turning round the higher di- 

 vided circle. Tlie glass plate covering the brass jar is surrounded 

 with leather, so that it can be arrested by means of a screw 

 passing through the brass case. By slackening this screw the 

 instrument may also be adjusted, but not so certainly as by turn- 

 ing the higher graduated circle; because by screwing up the 

 screw, the glass plate may too readily be displaced. On charging 

 the measurer, the arm of the balance must rest upon the brass strip 

 and form a right angle with it ; for this to be effected, it must 

 be turned to 90° and then lowered. The elevation and depres- 

 sion of the arm of the balance is effected by turning a screw, x, 

 which passes through a bracket, w, attached to the upper index- 

 plate. The end of this screw, formed into a peg, enters the top 

 of the brass rod upon which the glass thread is fastened, and is 

 held in its place by a transverse screw, c', which passes from the 

 exterior of the rod and works into a thin part cut in the peg. 

 By this arrangement the movement of the rod may be regulated 

 with great accuracy. 



After charging the instrument, the upper index is turned to 

 0° ; the arm of the balance below would therefore also move to 0° 

 if it had not become electrical. The angle which it makes with 

 the brass strip, after it has come to rest, is the magnitude from 

 which the electrical quantum may easily be calculated by means 

 of a table. The tension of one element of a zinc-copper battery 

 is taken as unity. The mean values obtained from three daily 

 observations for two years, give two mean yearly values which 

 nearly agree, namely 157-9 and 155-2, although the mean 

 monthly values for the two years differ considerably. The mean 

 monthly values, commencing with January, are for the first year, 

 191-5; 187-6; 150-9; 133-7; 114-2; 1229; 124-7; 130-8; 



142-4; 190-2; 172-6; 233-2: 

 and for the second, 

 169-5; 140-4; 150-6; 1284; 1141; 1191; 1187; 1493; 



154-1 ; 163-1 ; 226-2 ; 229-2. 



Kreutznach, February 1857- 



LVIII. On the Rotation of the Electrical Light round the Pole of 

 an Electro-magnet. By M. A. De la Rive*. 

 To Dr. Faraday, F.R.S. 8^-c. 

 My dear Friend, Geneva, May 10, 18.^)8. 



HAVING learnt from my friend Professor Marcct tliat you 

 and Mr. Gassiot have been occupied with the remarkable 

 luminous effects produced by the; electrical discharges of IJulnn- 

 * Communicated hv Professor Faraday. 



