244 U . S . BUREAU OF STANDARDS NOTES. [J. F. I.

THE ACTION OF CHARRED PAPER ON THE PHOTOGRAPHIC PLATE AND A METHOD OF DECIPHERING

CHARRED RECORDS.'

By Raymond Davis.

[ABSTRACT.]

THE Bureau of Standards was recently called upon to find a method for deciphering the written and printed matter contained on charred paper.

The charred papers submitted apparently had been subjected to heat in a closed vessel, such as a safe-- the paper having been converted into black sheets of carbon and not to ashes as would have been the case had they been burned in an open container.

With casual observation no traces of writing are visible, but under certain critical conditions of lighting very faint traces of markings can be seen. These traces are not sufficiently clear to permit deciphering.

In some preliminary trials made on paper charred for the purpose, conversion of the iron salts contained in the ink into colored salts was tried. These were unsuccessful.

It is known that the photographic plate, besides being sensitive to light, is also sensitive to certain gases and vapors. Some of these have the property of fogging or rendering developable such portions of the plate as are exposed to their actions. Certain other gases or vapors have the contrary property, that is, the 3 , partially or completely desensitize the plate.

For the first trial a sheet of the carbonized paper was placed between two " f a s t " photographic plates and kept in the dark for two weeks. On development in the usual manner a very perfect copy of both the writing and the printing was obtained. It appears that the carbonized paper contains gases that fog the photographic plate. Where the ink is present, little or no effect takes place. Apparently the ink acts as a screen, hindering the escape of the gas.

It is interesting to note that the writing on both sides of the charred paper appears, that from the back being fainter than that from the face. Apparently the ink penetrates the paper sufficiently so that its residue reduces the amount of gas escaping from beneath.

Scientific Papers, No. 454.

Feb., 1923.] U . S . BUREAU OF STANDARDS .NOTES. 245

No attempt was made to determine the nature of the active material contained in the charred paper. It is quite likely that it contains products similar to those obtained by the destructive distillation of wood.

Further tests showed that photographic plates of sensitiveness usually termed fast or medium are best adapted for this purpose. However, the sensitiveness to light is no definite indication of the sensitiveness to the charred paper. For example, Seed 30 and Seed 26X are of equal sensitiveness to the charred paper, the former is considerably faster to light than the latter. Very slow plates, such as " Process " are not suitable as exposure of thirtv- two days shows only faint action. Photographic printing and enlarging papers are very insensitive to the charred paper.

Results obtained by the use of photographic films were very surprising, as shown by the two types of emulsions selected for the test, namely, Eastman " Portrait Film " and Eastman " Super Speed Portrait Film."

The Portrait Film showed Ilo effect in thirty-two days. The Super Speed Portrait showed very slight but interesting effect with a sixteen-day exposure and only a little better at thirty-two days. This was just the reverse of that with the plates, the inked areas showing black on the films, whereas o11 the plates they show clear. That is, with the films the ink is the active portion, the charred paper produces no fogging. It was also noted that the chemical fog of development is much lighter over the portions of the film covered by the charred paper as compared with the uncovered areas.

The results suggest that there are perhaps two different kinds of gas given off, one kind by the charred paper and another by the ink, both of which fog the photographic emulsion, but the one from the charred paper more rapidly.

It was found that after a short washing these films in distilled water, for about five minutes, followed by thorough drying, gave results similar to those obtained with plates. Very good copies were obtained from washed film after eight days' contact with the charred paper.

Separate sheets of the charred paper which had been exposed to the air for several months required two weeks' contact with the photographic plate to produce a good printing negative, whereas sheets taken from a charred ledger required only one week to give

246 U . S . BUREAU OF STANDARDS NOTES. [J. F. I.

an equally good negative. It is natural to assume that the active material is gradually lost by exposure to a i r - - the activity having dropped one-half in about two months' time. Following this, however, the rate of depletion has dropped considerably, no fur- ther reduction in activity having been observed.

TABLES FOR THE CALCULATION OF THE INDUCTANCE OF CIRCULAR COILS OF RECTANGULAR CROSS-SECTION2

By Frederick W. Grover.

[ABSTRACT.]

MULTIPLE-LAYER coils are much used where a larger induc- tance is required than can conveniently be obtained from a single- layer coil. To reduce the considerable capacity which is associated wi th a simple circular coil wound in layers in a channel of rectangular cross-section, recourse is had to "banked " windings, or the layers may be spaced apart by small thicknesses of insulat- ing material, or a "honeycomb" type of winding may be employed.

In all these cases, the inductance may be calculated with a good degree of accuracy by the formulas which apply to a circular coil with the current uniformity distributed over a rectangular cross-section. For this purpose the dimensions of that ideal coil o f this type which is equivalent to the actual coil must be used. F o r the dimensions of cross-section there should be taken the product of the winding pitch in the layer by the number of turns .in the layer for one dimension, and for the other the product of the distance between corresponding turns in successive layers by the number of layers. The mean radius of the equivalent coil is the same as the mean radius of the actual coil. Thus, the rectangular cross-section of the equivalent coil may be imagined to be made up of a rectangular array of equal smaller" rectangles, with a turn of the wire of the coil at the centre of each. The inductance of the actual coil differs, of course, from that of the equivalent coil by a small amount, for which a correction may be calculated as shown by Rosa.

A number of formulas have been developed in the past for calculating the inductance of a circular coil of rectangular cross- sectioni Of these especial mention may be made of the formulas of Lyle, Dwight, and Butterworth, which between them cover

' Scientific Papers, No. 455.