138 



NATURE 



[June 9, 1898 



remained stationary. On withdrawing the hand, the rods took 

 up their former diverged position. 



This simple experiment demonstrated at once the excellent 

 insulating property of gutta percha and its inductive capacity. 

 Its insulating power was shown by the fact, that the tissue formed 

 an efficient screen between the hand and the brass disc of the 

 electroscope, to prevent the latter from being discharged. Its 

 inductive capacity was shown by the temporary fall of the pith 

 rods, indicating the " binding" of the charge on them when the 

 tissue was touched by the hand. 



Per se the insulation of gutta percha should be as high as 

 possible, and the inductive capacity as low as possible ; but 

 whereas the latter property is mostly associated with other good 

 qualities of the material, such is not always the case with a high 

 insulation. 





Fig. 2. — Softening temperature of gutta percha. 



Faraday apparently had some difficulty in 1848 in obtaining 

 gutta percha having a sufficiently good insulation. He found 

 that this was due to an excessive amount of water contained in 

 the commercial material. This is an important matter, and 

 experiments were shown by Dr. Obach to demonstrate the 

 effect of different percentages of water on the insulating power of 

 gutta percha. The electroscope was charged until the rods fully 

 diverged. Strips of gutta percha, containing approximately 15, 

 10, 5 and 2\ per cent, of water, were then successively brought 

 into contact with the brass knob, the finger being held against 

 the other side of the strip. When the strip containing 15 per 

 cent, of water was brought into contact with the knob, the pith 

 rods slowly converged, and did not regain their former position 

 on removing the strip, which showed that the charge had been 

 dissipated. On repeating the experiment with the next strip, con- 

 taining 10 per cent. , the charge disappeared much more slowly ; 



NO. 1493, VOL. 58] 



the strip containing 5 per cent, of water was next tried, and 

 this was found to be an almost perfect insulator and practically 

 equal to the best strip with 2\ per cent, of water. It must be 

 mentioned however, that different sorts of gutta percha behave 

 differently in this respect. The specific insulation and inductive 

 capacity of various specimens of gutta percha are given in a 

 table in comparison with other materials, such as paraffin wax, 

 colophony, ebonite, &c., but space does not permit us to repro- 

 duce this interesting taljle here. The figures show how greatly 

 the electrical data vary for different kinds of gutta percha. For 

 instance, the insulation resistance per cube knot was only 382 

 megohms for an otherwise excellent specimen of gutta percha, 

 and 139,300 megohms for a specimen of considerably inferior 

 description. Gutta percha extracted from leaves has usually a 

 comparatively high insulation, exceeding that of paraffin wax, 

 colophony and ebonite, but the insulation resistance of pure vul- 

 canised caoutchouc is higher still, approaching the maximum 

 obtained with ordinary gutta percha. The specific inductive 

 capacity also varies greatly, the lowest values per cube knot 



Earth 



Fig. 3.— Insulation and electrostatic capacity of gutta percha. 



being -0466 and the highest 'oSoi microfarad. In this respect 

 the gutta obtained from leaves by Dr. Obach's process compares 

 favourably with any ordinary gutta percha. Pure Para-caoutchouc 

 also has a very low inductive capacity, viz. "0446 mfd. , which is 

 lower than that of the best gutta percha, but paraffin wax is 

 lower still, viz. only •041 1 mfd. Water, on the other hand, has 

 the highest known inductive capacity of any substance, i.e. i'348 

 microfarad per cube knot. The significance of this will be seen 

 on comparing the values in Table II., which gives the insulation 

 and inductive capacity of several ' ' brands " of gutta percha, 

 each with a high and low percentage of water. 



The dielectric strength of insulating materials is another pro- 

 perty, which is daily becoming more important. From tests 

 made on gutta percha-covered cores of submarine cables, it 

 has been found that a thickness of \ inch of this dielectric is 

 pierced by about 40,000 volts, and one of yV inch by about 28,000 

 volts. 



The next table (III.), which is abstracted from a large table 

 in the Journal, contains the chemical composition and the 

 physical, mechanical and electrical properties of the first grades 

 of the twelve principal brands of gutta percha. 



