306 Prof. Faraday on Electric Induction — 



pens, so that it gave beautiful illustrations and records of facts 

 like those stated : the pens are iron wires, under which a band 

 of paper imbued with ferro-prussiate of potassa passes at a 

 regular rate by clock-work ; and thus regular lines of prussian 

 blue are produced whenever a cui-reut is transmitted, and the 

 time of the current is recorded. In the case to be described, 

 the three lines were side by side, and about O'l of an inch apart. 

 The pen in lielonged to a circuit of only a few feet of wire and 

 a separate battery ; it told whenever the contact key was put 

 down by the finger ; the pen n was at the earth end of the long 

 air wire', and the pen o at the earth end of the long subterraneous 

 wire ; and by arrangement, the key could be made to throw the 

 electricity of the chief battery into either of these wires, simul- 

 taneously with the passage of the short circuit current through 

 pen m. When pens m and n were in action, the m record was 

 a regular line of equal thickness, showing by its length the actual 

 time during which the electricity flowed into the wires ; and the 

 n record was an equally regular line, parallel to, and of equal 

 length with the former, but the least degree behind it ; thus 

 indicating that the long air wire conveyed its electric current 

 almost instantaneously to the further end. But when pens m 

 and were in action, the o line did not begin until some time 

 after the m line, and it continued after the m line had ceased, 

 i. e. after the o battery was cut off. Furthermore, it was faint 

 at first, grew up to a maximum of intensity, continued at that 

 as long as battery contact was continued, and then gradually 

 diminished to nothing. Thus the record o showed that the wave 

 of power took time in the water wire to reach the further extre- 

 mity ; by its first faintness, it showed that power was consumed 

 in the exertion of lateral static induction along the wire; by the 

 attainment of a maximum and the after equality, it showed when 

 this induction had become proportionate to the intensity of the 

 battery current ; by its beginning to diminish, it showed when 

 the battery current was cut off; and its prolongation and gradual 

 diminution showed the time of the outflow of the static electri- 

 city laid up in the wire, and the consequent regular falling of 

 the induction which had been as regularly raised. 



With the pens m and o, the conversion of an intermitting into 

 a continuous current could be beautifully shown ; the earth wire, 

 by the static induction which it permitted, acting in a manner 

 analogous to the fly-wheel of a steam-engine or the air-spring of 

 a pump. Thus, when the contact key was regularly but rapidly 

 depressed and raised, the pen m made a series of short lines 

 separated by intervals of equal length. After four or more of 

 these had passed, then pen o, belonging to the subterraneous 

 wire, began to make its mark, weak at first, then rising, to a 



