Page:Popular Science Monthly Volume 44.djvu/61

 pen directly, but in controlling the mechanism which actuates it. As the extent of movement of the pens is determined only by the number of electrical impulses, these may be given any desired range, and it becomes possible to use the transmitting pen with almost the same freedom as a pen or pencil in ordinary writing, and to write in the same way—that is, in successive lines extending across a page.

In the final form given to the instruments by Prof. Gray and shown at the exposition the transmitter consists of a box provided with a leaf upon which the paper rests. The paper is drawn continuously from a roll, and is shifted mechanically from time to time by the operator. The writing pen consists of a pencil lead mounted in a holder, to the lower end of which two silk threads are attached. These threads are at right angles to each other, and lead from

the pencil to two drums, upon which they are wound in such a manner as to cause the drums to rotate backward and forward as the threads follow the movement of the pencil point. The drums, therefore, move in exact accordance with the rectilinear components of the pen's motion, and it is only necessary to reproduce their motions at the other end to cause the receiving pen to duplicate the movements of the transmitting one. In an earlier form of the transmitter each drum carried an arm, which was swept by its movement over a series of radial electrical contacts, and thus sent a succession of electrical impulses to line. The friction of this moving arm was, however, found to be objectionable, and this arrangement has therefore been replaced by a magnetic device in which a toothed iron disk acting magnetically upon a soft iron lever keeps this in vibration. This lever plays between two contact points, and according as it is upon one or the other of the contacts a positive or negative current impulse is sent by a battery through the line circuit. These current impulses of alternating polarity serve to operate at the receiver polarized relays, which control by means of escapements drums similar to those in the transmitter, which drums actuate the receiving pen.

This pen consists of a glass tube drawn to a capillary bore at the end and supplied with a free-flowing ink from a reservoir by means of a rubber tube. It is mounted upon and at the junction