According to Edison (British Patent, No. 792, 1882), compressed graphite is a substance of _great conductivity_. According to Faraday (‘Exp. Res.’ vol. i. p. 24), retort-carbon is an _excellent conductor_. Both graphite and retort-carbon agree with the metals in the property that the electric resistance offered at a point of contact between them varies when the pressure at the contact is varied. It is indeed remarkable through what wide ranges of resistance the contact between two good conductors may vary. The resistance of contact between two pieces of copper may be made to vary in a perfectly continuous manner by changes of pressure through a range, according to Sir W. Thomson, from a small fraction of one _ohm_, up to a resistance of many thousand _ohms_. The same is true of silver, brass, and many other good conductors, including graphite and retort-coke, though with the latter materials the range of resistances is not so great. With partial conductors, such as oxide of manganese, sulphide of copper, sulphide of molybdenum, &c., and with bad conductors, such as lamp-black and selenium, whose conductivity is millions of times less than that of graphite, copper, and other good conductors, it is impossible to get equally wide variations of resistance, as the amount of pressure at a point which will bring the bad conductors into intimacy of contact, will not turn them into good conductors. Platinum being in the category of good conductors, is amongst those substances which yield a very wide range of electrical resistances at the contact-points which are submitted to varying pressures. With the very highest conductors, such as silver and copper, the electrical range of contact-resistance is higher than with those of lesser conductivity, such as lead, platinum, graphite, and retort-coke. But though the range of variation in electrical resistance at contacts is highest for the best conductors, there comes in another element, namely, the range of distance through which the contact-pieces, or either of them, must be moved in order to pass through the range of variations of resistance. This is quite a different matter, for here the best conductors have the smallest range, and some that are not so good a greater range. In any case the available range of motion is very small--to be measured in minute fractions,--millionth-parts, perhaps,--of an inch. So far as experiments go, however, silver has the smallest range of all, then gold, then copper. Platinum and nickel have a considerably wider range, plumbago and retort-coke a still wider one. It is an extremely difficult matter to decide what is the precise nature of that which goes on at a point of contact between two conductors when the pressure at the point is altered. The principal suggestions hitherto advanced have been that the change of resistance observed is due:-- (_a_) To the mere changes in the amount of surface in contact.