214 WRITINGS OP JOSEPH HENRY. [1843 



In order that the pen may not describe a line on the 

 cylinder, re-entering into itself, and thus obliterate the dot 

 first impressed, it may be found necessary to give the 

 cylinder a slow ascending motion, so that a spiral instead of 

 a circle would be marked on its surface. A chronometer for 

 measuring minute portions of time, with a motion of this 

 kind is described in Young's Natural Philosophy, vol. i, 

 page 191. 



To prevent agitations of the air, the whirling apparatus 

 with the galvanometer may be placed in the vacuum of an 

 air pump; and that part of the conducting wire which 

 crosses the screen may be separated at each crossing, the 

 ends being again united by slightly twisting them together, 

 and the conduction being preserved by proper amalgama- 

 tion, so that the force necessary to break the circuit may not 

 sensibly lessen the velocity of the ball. 



Various other methods may be devised for impressing a 

 mark on the revolving cylinder, at the moment of the 

 rupture of the galvanic current by the passage of the ball 

 through the screen. But the following, which has suggested 

 itself to Professor Henry since the meeting of the Society, 

 and has been communicated by him to the Reporter, may 

 be regarded as among the best. It dispenses with the gal- 

 vanometers, and produces the mark by a direct electrical 

 action. 



A part of the long wire which leads to the screen is coiled 

 around a bundle of soft iron wire; and over this is coiled 

 another long wire, so as to produce an intense secondary 

 current, on the j)rinciple of the common coil machine. One 

 extremity of the secondary circuit is connected with the 

 axis of the cylinder, and the other is made to terminate 

 almost in contact with the revolving surface, which in this 

 modification of the instrument is surrounded by a ruled or 

 graduated paper. It is obvious that the secondary current 

 which is induced by the interruption of the primary circuit, 

 will pierce or mark the paper band at the moment of the 

 screen being broken. There is no difhculty in effecting 



