The late Professor Faraday has proved that one cell of a Voltaic pile possesses the same quantity of electricity as an ordinary sized frictional machine after being wound round 800,000 times, thus showing the contrast between the qualities of frictional and Voltaic electricity. The electricity of the frictional machine and that of the galvanic battery may be made to produce the same effect, there being no difference in kind between them. Frictional electricity can be made to pass in a current, but it is comparatively feeble. Again, Voltaic electricity can be made to produce a spark, but under ordinary circumstances it scarcely amounts to anything. _Description of a Frictional Electric Machine._--A frictional electrical machine consists of a vulcanite or glass disc or cylinder, which is made to revolve between cushions or rubbers of leather or silk. By the friction the (silk) rubbers become negatively, and the glass disc or cylinder positively, electrified. The revolving disc immediately after contact with the fixed rubbers passes close by a series of brass points, which are connected with a _condenser_. These points collect the positive electricity of the glass, the rubbers being put to earth. The positive electricity which the glass loses is supplied through the rubber; a stream of negative electricity flows from the rubbers to the earth during the charging of the conductor or condenser; in other words, the positive electricity flows from the earth to the rubber, whence it crosses to the glass disc and so to the condenser. _Definition of a "Condenser."_--A _condenser_ is an arrangement for accumulating a large quantity of electricity on a comparatively small surface. _The "Leyden Jar."_--The _Leyden jar_, which is the original type of the condenser, or accumulator, consists of a glass jar coated inside and out, up to within a few inches of the mouth, with tinfoil pasted on, but having no connection with each other. The mouth is usually closed by means of a wooden stopper, through which a brass rod passes, to the head of which is affixed a brass knob, &c., the rod and knob being metallically connected with the _inner_ coating by means of a chain. The "Leyden jar" may be charged either by connecting the _outer_ coating to earth (the rubbers of the machine being also to earth), and the _inner_ coating to the conductor of the machine; or else by connecting the outer coating to the rubbers, and the inner coating to the conductor, a complete circuit being necessary to charge the jar as highly as the frictional electrical machine will admit of. The _conductor_ of the machine being charged, also forms a kind of Leyden jar, the conductor in this case being the inner coating, the air, the _dielectric_, and the nearest surrounding conductors, such as the walls of the room, &c., being the outer coating. _Meaning of "Dielectric."_--By _dielectric_ is meant a non-conducting medium, which in the case of the "Leyden jar" is the glass.