(6) The "contact" resistance between slabs of wood pressed together is always very high.
The following table will sufficiently illustrate the results obtained. The stone was dried in the sun for three weeks in the summer (United States), and the wood is described as having been well seasoned:—
CURRENT WITH THE GRAIN
| Lowest Resistance between two Cups in Megohms. | Highest Resistance between two Cups in Megohms. | Lowest Specific Resistance in Megohms. | Highest Specific Resistance in Megohms. | |
| Ash. | 550 | 920 | 380 | 700 |
| Cherry | 1100 | 4000 | 2800 | 6000 |
| Mahogany | 430 | 730 | 310 | 610 |
| Oak | 220 | 420 | 1050 | 2200 |
| Pine. | 330 | 630 | 360 | 1470 |
| Hard pine. | 10 | 48 | 17 | 1050 |
| Black walnut | 1100 | 3000 | 320 | 2100 |
| Red fibre | 2 | 4 | 3 | 60 |
| Slate | 184 | 280 | ||
| Soapstone. | 330 | 500 | ||
| White marble | 2000 | 8800 |
§ 115. As to working the materials very little need be said.
Fibre is worked like wood, but has the disadvantage of rapidly taking the edge off the tools. In turning it, therefore, brass-turning tools, though leaving not quite such a perfect finish as wood-turning tools, last much longer, and really do well enough. Fibre will not bear heating much above 100° C. — at all events in paraffin. At 140° C. it becomes perfectly brittle. Its chief merit lies in its great strength. So far as insulation is concerned, Mr. Peirce's experiments show that it is far below most kinds of wood.
Slate. — This is a vastly more useful substance than it is generally credited with being. It is very easily worked at a slow speed, either on the shaping machine or on the lathe, with tools adjusted for cutting brass, and it keeps its figure, which is more than can be said for most materials. It forms a splendid base for instruments, especially when ground with a little emery by iron or glass grinders, fined with its own dust, and French polished in the ordinary way. Spools for coils of considerable radial dimension may be most conveniently made of slate instead of wood or brass, both because it keeps its shape, and because it insulates sufficiently well to stop eddy currents — at all events, sufficiently for ordinary practice. An appreciable advantage is that slate may be purchased at a reasonable rate in large slabs of any desired thickness. It is generally cut in the laboratory by means of an old cross-cut saw, but it does not do much damage to a hard hack saw such as is used for iron or brass.
Marble. — According to Holtzapffell, marble may be easily turned by means of simple pointed tools of good steel tempered to a straw colour. The cutting point is ground on both edges like a wood-turning tool, and held up to the work "at an angle of twenty or thirty degrees" (?with the horizontal). The marble is cut wet to save the tool. As soon as the point gets, by grinding, to be about one-eighth of an inch broad it must either be drawn down or reground; a flat tool will not turn marble at all.
A convenient saw for marble is easily made on the principle of the frame saw. A bit of hoop iron forms a convenient blade, and is sharpened by being hammered into notches along one edge, using the sharp end of a hammer head. The saw is liberally supplied with sand and water — or emery and water, where economy of time is an object. The sawing of marble is thus really a grinding process, but it goes on rapidly. Marble is ground very easily with sand and water; it is fined with emery and polished with putty powder, which, I understand, is best used with water on cloth or felt. As grinding processes have already been fully described, there is no need to go into them here. I have no personal knowledge of polishing marble.