THE ELECTRIC TELEGRAPH. 



the roller. This roller is itself kept in uniform revolution by 

 means of a train of wheel- work, deriving its motion either from a 

 weight or main spring, and regulated by a governor or fly. The 

 rate at which the disk revolves may be varied at the discretion of 

 the superintendent, by shifting the position of the roller towards 

 the centre ; the nearer to the centre the roller is placed, the 

 more rapid will be the motion of rotation. The moistened paper 

 being placed on this disk, we have a circular sheet kept in uniform 

 revolution. 



The electro-chemical pen, already described, is placed on this 

 paper at a certain distance from its centre. This pen is sup- 

 ported by a pen-holder, which is attached to a fine screw ex- 

 tending from the centre to the circumference of the desk in the 

 direction of one of its radii. 



On this screw is fixed a small roller, which presses on the 

 surface of the desk, and has sufficient adhesion with it to receive 

 from it a motion of revolution. This roller causes the screw to 

 move with a slow motion in a direction from the centre to the 

 circumference, carrying with it the electro-chemical pen. "We 

 have thus two motions, the circular motion carrying the moistened 

 paper which passes under the pen, and the slow rectilinear motion 

 of the pen itself directed from the centre to the circumference. 

 By the combination of these two motions, it is evident that the 

 pen will trace upon the paper a spiral curve, commencing at a 

 certain distance from the centre, and gradually extending 

 towards the circumference. The intervals between the successive 

 coils of this spiral line will be determined by the relative velocities 

 of the circular desk, and of the electro-chemical pen. The relation 

 between these velocities may likewise be so regulated, that the 

 coils of the spiral may be as close together as is consistent with the 

 distinctness of the traces left upon the paper. 



A view of the circular desk, the chemical pen, and the clock- 

 work is given in fig. 87 (p. 65), which will render the preceding 

 explanation more easily understood. 



43 



