| Ferrocyanide of potassium | 1/4 oz. |
| Ammoniac Nitrate | 1/2 oz. |
| Distilled water[[8]] | 4 oz. |
The paper has to be very carefully chosen, as besides being absorbent enough to remain moist during the whole of the receiving, the surface must also remain fairly smooth, as with a rough paper the grain shows very distinctly, and if there is an excess of solution the electrolytic marks are inclined to spread and so cause a blurred image. The writer tried numerous specimens of paper before one could be found that gave really satisfactory results. It was also found that when working in a warm room the paper became nearly
dry before the receiving was finished, and the resistance of the paper being greatly increased (this may be anything up to 1000 ohms), the marking became very faint. A sponge moistened with the solution and applied to the undecomposed portion of the paper, while still revolving, was found to help matters considerably.
Another experience which happened during the writer's early experiments, the cause of which I am still unable to explain, occurred in connection with the stylus. The stylus used consisted of a sharply pointed steel needle, and after working for about three minutes it was noticed that the lines were becoming gradually wider, finally running into each other. Upon examination it was found that the point of the needle had worn away considerably, becoming in fact, almost a chisel point. Almost every needle tried acted in a similar manner, and to overcome this difficulty the stylus shown in Fig. 31 was devised.
It will be seen that it consists of a holder A, somewhat resembling a drill chuck, fastened to the flat spring B in such a manner that the angle the stylus makes to the drum can be altered. The needle consists of a length of 36-gauge steel wire, and as this wears away slowly the jaws of the holder can be loosened and a fresh length pushed through. The wire should not project beyond the face of the holder more than 1/8th inch. The gauge
of wire chosen would not suit every machine, the best gauge to use being found by trial, but in the writer's machine the pitch of the decomposition marks is much finer than of those made by the commercial machines, and this gauge, with the slight but unavoidable spreading of the marks, will produce a mark of just the right thickness. As already mentioned, no explanation of this peculiarity on the part of the stylus can be given, as there is nothing very corrosive in the solution used, and the pressure of the stylus upon the paper is so slight as to be almost negligible.
No special means are required for fastening the paper to the drum, the moist paper adhering quite firmly. Care should be taken, however, to fasten the paper—which should be long enough to allow for a lap of about 1/4 inch—in such a manner that when working the stylus draws away from the edge of the lap and not towards it.
The current required to produce electrolysis is very small, about one milliampere being sufficient.