THE GALVAXOGRAPH 201 



circuit ; also the discharge of the spark gives a backward 

 kick to the index, by which the normal deflection undergoes 

 an unknown variation. 



These difticulties were removed in the following manner: 

 The moving coil of the sensitive D'Arsonval galvanometer 

 has a long glass index I at right angles to the plane of the 

 coil, the index being coated with shellac varnish to render 

 it highly insulating. It projects for a short distance on 

 the opposite side for attachment of a counterpoise, which 

 takes the form of a vertical vane of mica acting as a damper. 

 The galvanometer itself is of an aperiodic type, and the 

 addition of the damper makes it perfectly dead-beat. The 

 sensitiveness of the galvanometer is such that a micro- 

 ampere current produces a deflection of lo mm. of the index. 

 The recording index has attached to it a short vertical piece 

 of thin platinum wire pointed at its two ends ; the index 

 moves between the sheet of metal M, covered with paper for 

 record, and a semicircular piece of narrow sheet-metal C. 

 The metal sheet M is mounted on wheels and moves at a 

 uniform rate by clockwork. One electrode of the sparking 

 coil is in connection with C, and the other with M. The 

 sparking thus takes place simultaneously, above and below 

 the vertical and double-pointed platinum wire carried at 

 the end of the index. There is thus no resultant kick, and 

 the index remains undisturbed. The sparking, as previously 

 stated, takes place 3 seconds after exposure of the selenium 

 cell to light, by which time the deflection reaches its 

 maximum. The record thus consists of successive dots 

 at intervals of 15 minutes, the dots representing the 

 maximum deflections of the galvanometer corresponding 

 to the intensity of light. 



The record given in fig. 116 was taken about the end of 

 January ; the sun rose at about 6.45 a.m. and set at 5.30 p.m. 

 Twilight is very short in the tropics ; the sky is feebly 

 lighted about 6 A.M. The record shows the intensity of 

 light to be exceedingly feeble at 6 a.m. The rise in the 

 intensity was rapid, attaining the maximum at 12 midday. 



