§ 114 Imperfect Conductors.
Under this heading may be grouped such things as wood, slate, marble, etc. — in fact, materials generally used for switchboard insulation. An examination of the insulating power of these substances has recently been made by B. O. Peirce (Electrical Review, 11th January 1895) with quite sufficient accuracy, having in view the impossibility of being certain beforehand as to the character of any particular sample. The tests were made by means of holes drilled in slabs of the material to be examined. These holes were three-eighths of an inch in diameter, and from five-eighths to three-quarters of an inch deep, and one inch apart, centre to centre. A voltage of about 15 volts was employed. The following general results were arrived at:-
(1) Heating in a paraffin bath always increases the resistance of wood, though only slightly if the wood be hard and dense.
(2) Frequent exhaustion and readmission of air above the surface of the paraffin always has a good effect in increasing the resistance of wood.
(3) When wood is once dry, impregnating it with paraffin tends to keep it dry.
(4) Red vulcanised fibre, like wood, absorbs paraffin, but it cannot be entirely waterproofed in this way.
(5) The resistance of wood with stream lines along the grain is twenty to fifty per cent lower than when the stream lines cross the grain.
(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.
The properties of conductors, more particularly of metals, have been so frequently examined, that the literature of the subject is appallingly heavy. In what follows I have endeavoured to keep clear of what might properly appear in a treatise on electricity on the one hand, and in a wiring table on the other. The most important work on the subject of the experimental resistance properties of metals has been done by Matthieson, Phil. Trans. 1860 and 1862, and British Association Reports (1864); Callender, Phil. Trans. vol. clxxiii.; Callender and Griffiths, Phil. Trans. vol. clxxxii.; The Committee of the British Association on Electrical Standards from 1862 to Present Time; Dewar and Fleming, Phil. Mag. vol. xxxvi. (1893);
Klemencic, Wiener Sitzungsberichte (Denkschrift), 1888, vol. xcvii. p. 838; Feussner and St. Lindeck, Zeitsch. fuer Inst. ‘Kunde, ix. 1889, p. 233, and B. A. Reports, 1892, p. 139. Of these, Matthieson, and Dewar and Fleming treat of resistance generally, the latter particularly at low temperatures.
Matthieson, and Matthieson and Hockin, Klemencic, Feussner, and St. Lindeck deal with the choice of metals for resistance standards. Callender’s, and Callender and Griffiths’ work is devoted to the study of platinum for thermometric purposes.
The bibliography referring to special points will be given later. The simplest way of exhibiting the relative resistances of metals is by means of a diagram published by Dewar and Fleming (loc. cit.), which is reproduced on a suitable scale on the opposite page. For very accurate work, in which corrections for the volumes occupied by the metals at different temperatures are of importance, the reader is referred to the discussion in the original paper, which will be found most pleasant reading. From this diagram both the specific resistance and the temperature coefficient may be deduced with sufficient accuracy for workshop purposes. In interpreting the diagram the following notes will be of assistance. The diagram is drawn to a scale of so-called “platinum temperatures” — that is to say, let R0, R100, Rt be the resistances of pure platinum at 0°, 100°, and t° C. respectively, then the platinum temperature pt is defined as pt = 100 X (Rt-R0)/(R100-R0)
This amounts to making the temperature scale such that the temperature at any point is proportional to the resistance of platinum at that point. Consequently on a resistance temperature diagram the straight line showing the relation between platinum resistance and platinum temperature will “run out” at the platinum absolute zero, which coincides more or less with the thermodynamic absolute zero, and also with the “perfect gas” absolute zero. Platinum temperatures may be taken for workshop purposes over ordinary ranges as almost coinciding with air thermometer temperatures. The metals used by Professors Dewar and Fleming were, with some exceptions, not absolutely pure, but in general represent the best that can be got by the most refined process of practical metallurgy. We may note further that the specific resistance is only correct for a temperature of about 15° C., since no correction for the expansion or contraction of material has been applied.
The following notes on alloys suitable for resistance coils will probably be found sufficient.
This substance, discovered by Martino and described by Bottomley (Phil. Proc. Roy. Soc. 1885), is an alloy of nickel, zinc, copper, and 1 per cent to 2 per cent of tungsten, but I have not been able to obtain an analysis of its exact composition. It appears to be difficult to get the tungsten to alloy, and it has to be added to part of the copper as phosphide of tungsten, in considerably greater quantity than is finally required. The nickel is added to part of the copper and the phosphide of tungsten, then the zinc, and then the rest of the copper. The alloy requires to be remelted several times, and a good deal of tungsten is lost by oxidation.
The alloy is of a fine white colour, and is very little affected by air — in fact, it is to some extent untarnishable. The specific resistance will be seen to be about one and a half times greater than that of German silver, and the temperature coefficient is about 0.021 per cent per degree C. (i.e. about nineteen times less than copper, and half that of German silver). To all intents and purposes it may be regarded as German silver with 1 per cent to 2 per cent of tungsten. It does not appear to have been particularly examined for secular changes of resistance.
118. German Silver. — This material has been exhaustively examined of late years by Klemencic and by Feussner and St. Lindeck. Everybody agrees that German silver, as ordinarily used for resistances, and composed of copper four parts, zinc two parts, nickel one part, is very ill-fitted for the purpose of making resistance standards. This is due
(1) to its experiencing a considerable increase in resistance on winding. Feussner and St. Lindeck found an increase of 1 per cent when German silver was wound on a core of ten wire diameters.
(2) To the fact that the change goes on, though with gradually decreasing rate, for months or years;
(3) to the fact that the resistance is permanently changed (increased) by heating to 40° C. or over. By “artificially ageing” coils of German silver by heating to 150° C., say for five or six hours, its permanency is greatly improved, and it becomes fit for ordinary resistance coils where changes of, say, 1/5000 do not matter.
It is a remarkable property of all nickel alloys containing zinc that their specific resistance is permanently increased by heating, whereas alloys which do not contain zinc suffer a change in the opposite direction. The manufacturers of German silver appear to take very little care as to the uniformity of the product put on the market; some so-called German silver is distinctly yellow, while other samples are bright and white.
It is noted by Price (Measurements of Electrical Resistance, p. 24) that German silver wire is apt to exhibit great differences of resistance within quite short lengths. This has been my own experience as well, and is a great drawback to the use of German silver in the laboratory, for it makes it useless to measure off definite lengths of wire with a view to obtaining an approximate resistance. In England German silver coils are generally soaked in melted hard paraffin. In Germany, at all events at the Charlottenburg Institute, according to St. Lindeck — coils are shellac-varnished and baked. In any case it appears to be essential to thoroughly protect the metal against atmospheric influence.
Notes
1 The following is a list of Dr. Matthieson’s chief papers on the subject of the electrical resistance of metals and alloys: Phil. Mag. xvi. 1858, pp. 219-223; Phil. Trans. 1858, pp. 383-388 Phil. Trans. 1860, pp. 161-176; Phil. Trans. 1862, pp. 1-27 Phil. Mag. xxi. (1861), pp. 107-115; Phil. Mag. xxiii. (1862), pp. 171-179; Electrician, iv. 1863, pp. 285-296; British Association Reports, 1863, p. 351.