TEANSACTIONS OF SECTION G. 777 



MONDAY, SEPTEMBER 5. 



The following Papers were read : — 

 1, On Telegraphic PJiotographij . By Shelford Bidwell, M.A., LL.B. 



The apparatus now explained is a development of that described in ' Nature,' 

 February 10, 1881, and exhibited at a meeting of the Physical Society on 

 February 26. Its purpose is to illustrate a method of transmitting pictui-es of 

 natural objects by telegraph. 



The receiving instrument contains a platinum-covered brass cylinder, 2 inches 

 long and | inch in diameter, mounted horizontally upon a spindle 7 inches long. 

 One of the projecting ends of the spindle has a screw cut upon it of 64 threads to 

 the inch ; the other end is left plain. The spindle revolves in two bearings, the 

 distance between which is equal to twice the length of the cylinder ; and one of 

 the bearings has an inside screw corresponding to that upon the spindle. A 

 platinum point, attached to an elastic brass arm, which is fixed midway between 

 the two bearings, presses normally upon the surface of the cylinder. The positive 

 pole of a local battery is connected through a set of resistance coils with the 

 platinum point, and the negative pole with the metal cylinder. If now a piece of 

 paper which has been soaked in a solution of iodide of potassium be wrapped 

 round the cylinder, and the cylinder be caused to rotate, the platinum point will, so 

 long as the current passes, trace a veiy close spiral brown line upon the paper. 

 When the strength of the current is diminished, the intensity of the line is 

 enfeebled ; when the current is interrupted the line is broken off, and the paper 

 retains its original whiteness. It is evident that by regulating the intensity of the 

 line thus traced, and introducing gaps in the proper places, any design or picture 

 might be represented upon the paper. This is the system adopted in Bakewell's 

 well-knovra copying telegraph. The author's instrument, however, differs from 

 his in that the current is varied simply by the action of light. 



At the transmitting station is a second battery, which is connected by two 

 line wires (one of which would, in practice, be replaced by the earth) with the 

 receiving instrument, so that the cmTeut passes through the prepared paper in the 

 opposite direction to that of the current from the local battery. If the two 

 currents through the paper are of equal strength they will neutralise each other, 

 and the platinum point wiU make no mark when the cylinder rotates : if the 

 current from the local battery is stronger than that from the other, the usual 

 brown mark will appear, its intensity depending upon the difference between the 

 ■Strength of the two currents. 



In the circuit of the second battery is introduced the transmitting instrument, 

 the purpose of which is to regulate the current in such a manner as to produce 

 the desired effects. The action of the transmitter depends upon the ciuious 

 property of crystalline selenium, that its electrical resistance is greater in the 

 dark than in the light. The current is caused to pass through a selenium cell, 

 enclosed in a small rectangular box, from which all light is excluded, except such 

 as can pass through a pinhole, drilled in the side of the box opposite to the cell. 

 By means of a mechanical arrangement, which, though very simple, cannot easily 

 "be described without a drawing, the box is connected with a horizontal spindle in 

 such a manner that each revolution of the spindle causes the box to move 

 perpendicularly up and down, through a distance of 2 inches, and at the same 

 time laterally through ~ inch. If now a picture, not more than 2 inches square, 

 is projected by a photographic lens upon that side of the box which contains the 

 pinhole, it is clear that by turning the spindle the pinhole may be caused to pass 

 successively over every point of the focussed image. The box, while moving in the 

 upward direction, travels through space at precisely the same rate as a point on the 

 surface of the receiving cylinder, when the spindles of the two instruments are 

 revolving synchronously : the downward movement is rapid and is not concerned in 

 the transmission of the picture. 



