[patteeson] self recording ELECTROMETER 19 



The quadrants are large enough to permit the use of 4^ vanes 

 and the faces of the plates are Y' apart.* The recording mechanism 

 is shown in plate II, fig. 3. Just below the boom is an inked thread 

 or carbon paper parallel to the axis of the paper roller L. The bar J. 

 which is operated by a cam attached to the driving mechanism is allowed 

 to drop once every two minutes and in doing so depresses the boom 

 causing the inked thread to strike the recording paper just below the 

 boom, giving a dot which shows the deflection of the needle at the 

 instant of marking. The boom being pivoted as already described 

 does not disturb the needle or bend the suspension. The inked thread 

 gives a short dash instead of a dot on the recording paper, and this 

 is not as satisfactory as it might be when the potential is constantly 

 changing. To obtain a dot instead of a dash, a narrow ribbon of 

 ordinary carbon paper was employed for a time, and a slotted bar 

 with a rod 1/16" diameter resting in the slot had to be substituted for 

 the roller L. when the boom was depressed it hit the carbon paper 

 just above the rod, and left a small dot. This arrangement was very 

 good as long as the room was dry, but if it were damp the boom was very 

 liable to stick on the paper. The difficulties were finally overcome 

 by taking a couple of threads out of a typewi'iter ribbon, twining them 

 together and then doubling. This was used in the same way as the 

 inked thread and gave good results, f It is a very simple matter to make 

 sufficient thread to last for several months without requiring attention. 

 The thread was stored on a spool and was wound off at the rate of 

 ly per day on to another spool. 



Clocks. 



There are two separate clocks in the clock case, the drum clock 

 and slave clock. The drum clock drives the paper at a definite speed 

 of 14 mm, per hour or 168 mm. per hour according to whichever speed 

 is required, generally the slower speed is used. The slave clock drives the 

 cam wheel which operates the depressor bar at intervals of two minutes. 



Charts. 



A continuous paper movement is provided. The paper is stored 

 on the drum P. plate II, fig. 3, and passes up at the back of the drum Q, 



* The faces of the plates were at first only 3/16" apart, but it was found that 

 when the potential on the needle exceeded 200 volts the attraction between the 

 plates and the vanes of the needle was sufficient to cause the vanes to touch the 

 plates and discharge, owing to the impossibility of adjusting them exactly parallel 

 to the faces of the plates. The plates were then put l" apart, and the trouble ceased. 



fSince going to press it was discovered that as the threads out of a typewriter 

 ribbon were fuzzy the hairs sometimes prevented the boom from registering prop- 

 erl5^ This was remedied bj' using fine sewing silk inked by rolling it in a type- 

 writer ribbon and putting them in a press for a day or two. 



