THE GALVANOGRAPH 103 



descends at a uniform rate by clockwork. This, however, 

 would entail the use of a dark room and subsequent develop- 

 ment of the plate. The trouble is avoided by the direct 

 record of the galvanometer deflection by means of electric 



sparks. 



A sparking method had been previously employed in 

 which the deflected index of the galvanometer in connec- 

 tion with one electrode of an induction coil leaves a spark 

 record on a moving sheet of paper. Several difficulties were, 

 however, encountered in the employment of this method 

 with a highly sensitive galvanometer. There is a liability 

 to leakage of the high-tension current into the galvano- 

 circuit. Secondly, the discharge of the spark gives a 

 backward kick to the index by which the normal deflection 

 undergoes an unknown variation. 



These difficulties were overcome in the following manner. 

 The moving coil of a sensitive D'Arsonval galvanometer 

 bears a long glass index I, at right angles to the plane 

 of the coil. The glass index is coated with shellac varnish 

 to render it highly insulating. The index is prolonged to a 

 short distance on the opposite side, for the attachment of 

 a counterpoise ; this takes the form of a vertical vane of 

 mica which acts 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 galvano- 

 meter is such that a micro-ampere of current produces a 

 deflection of 10 mm. of the index. The recording index 

 has attached to it a short vertical piece of thin platinum 

 wire pointed at its two ends ; this end of the index moves 

 between a sheet of metal M and a narrow semicircular 

 strip of metal c. The metal sheet M, bearing the sheet of 

 thin paper p for the record, is mounted on wheels and moves 

 at a uniform rate by clockwork. Record is made by sparks : 

 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 



