To find the percentage of conductivity in a sample of wire, pure copper being taken as 100.

Divide the resistance of 1 mile of pure copper wire of the same size (column 6), by the actual resistance of 1 mile of the wire tested (reduced to 32° Fah.), and multiply by 100. This table is used by Elliot Brothers.

No. 8 is the most convenient for stretching easily and making a neat job.

§ V.-a. GUTTAPERCHA.

1. The Specific Gravity of guttapercha is between 0.9693 and 0.981.

2. One cubic foot weighs between 60.56 and 61.32 lbs.; therefore, one cubic inch weighs between 0.560 and 0.567

ounces.

3. Guttapercha softens at 115° Fah., becomes plastic at 120°, melts at 212°, and is fluid at about 266°. When exposed to the air, particularly at temperatures between 70 and 90° Fah., it oxidises, becomes brittle, shrinks, and cracks.

This is very noticeable in guttapercha leads, used at telegraph offices in tropical climates.

Exposure to light hastens oxidation.

Oxidation and decay are hindered by serving guttapercha core with tape soaked in Stockholm tar; but the natural acid of the tar must first be removed, otherwise the insulation will be reduced. Guttapercha leads encased in lead tubing remain perfect for years. No deterioration is observable in guttapercha if kept continually under water.

4. Stretched Guttapercha, as used for core, resists a strain of 1000 lbs. per square inch before permanent elongation takes

place, and breaks at about 3500 lbs. per square inch. Stretching increases the tenacity of guttapercha in the direction stretched. D2-d2

5. The weight of guttapercha per knot in any core is 491 where D = diameter of guttapercha, and d = diameter of copper, (both in mils.)

6. The exterior diameter of any guttapercha core

where w is the weight in lbs. per knot of copper strand, and W that of the guttapercha. With a solid conductor the diameter

7. The electro-static capacity per nautical mile of any guttapercha core is approximately

8. The electro-static capacity of guttapercha core, as compared with indiarubber core of similar size, is as 120 to 100 nearly. 9. The Insulation-resistance per knot of a guttapercha core of the best quality=769 (Log D-log d) megohms at 75° Fah.

10. The resistance of guttapercha under pressure increases in the following ratio* :—

Let R be the resistance at atmospheric pressure, the resistance under the pressure p lbs. per square inch

=R (1+0.00023 p).

11. The resistance of guttapercha diminishes as the temperature increases; the rate of decrease is as follows* :

Let R= resistance at the higher temperature;

r = resistance at the lower temperature;

t = the difference of temperature in degrees Fah.,—then

log of R = log r―t log 0·9399, and log of r = log R + t log 0·9399.

Tables of resistance of guttapercha at different temperatures are given on the following page. (Table VI.)

* When a submarine cable is laid in deep water, the insulation is greatly improved owing to the pressure of the water and lower temperature (see Table XI.).

TABLE VI.—THE RELATIVE RESISTANCE (AFTER 1 MINUTE) OF ORDINARY GUTTAPERCHA AT DIFFERENT TEMPERATURES, FOR ALL CORES IN WHICH THE THICKNESS OF GUTTAPERCHA DOES

NOT EXCEED 0.11 INCH.-(W. Smith.) *

* From "Electrical Tables," Clark and Sabine.

The above table is for new core, as guttapercha improves greatly with age. For example :-The resistance at 32° Fah. is 23 622 times greater than at 75° Fah. Therefore, in order to reduce the observed resistance at any temperature to what it will be at 75° Fah., refer to the above table, and divide the observed resistance by the number in the relative resistance column, found opposite the temperature of the G.P. at time of test.-A. J.

12. A plate of guttapercha, 1 square foot surface and 1 mil. thick, has a resistance = 1066 megohms at 75° Fah.; and its electro-static capacity =·1356 microfarads.

13. The insulation resistance, in megohms, at 75° Fah., of an ordinary guttapercha cable or condenser, multiplied by its electro-static capacity, in microfarads = 144·4.

*Where D= diameter of core, and d-diameter of wire or mean diameter of strand, both in mils.; W = weight of guttapercha, and w= weight of conductor, both in lbs.

6. WILLOUGHBY SMITH'S IMPROVED GUTTAPERCHA.+

15. The specific gravity of guttapercha prepared by Mr. Willoughby Smith's process is the same as that of ordinary guttapercha.

16. The mechanical strength of this material is about 12 per cent. greater than that of ordinary guttapercha.

17. The electro-static capacity (F) per knot of a core of Smith's guttapercha is approximately

*For table to find resistance (after 1 minute) and capacity per knot of any ordinary guttapercha core from relative weights or diameter of insulator D ,—see Clark and Sabine, "Electrical Tables," page 122 conductor d'

† Also page 124.