ELECTRICAL ENGINEERING CHAPTER I. CURRENT POTENTIAL-CONDUCTORS-INSULATORS WHEN a stick of sealing-wax is rubbed with a piece of dry fur or flannel, the wax acquires the power of attracting to itself any light. substances that may be in its vicinity. By taking suitable precautions a like power can be detected in the fur or flannel. Similar phenomena can also be produced by the rubbing together of other substances, such as glass and silk, india-rubber and silk, brown paper and a bristle clothes-brush. A body which exhibits this power of attraction is said to be endowed or charged with electricity, or to be electrified. But there are two electrical states, and this can be easily proved; for if by means of a foot or so of silk ribbon we suspend the electrified sealing-wax and bring near it another electrified stick of sealing-wax, repulsion ensues-that is to say, the suspended rod recedes from the approaching one. On the other hand, if certain necessary precautions have been taken to prevent the neutralisation or escape of the electricity that was generated on the fur or flannel rubber, it will be seen that, on bringing it near the suspended sealing-wax, attraction takes place. A similar result would follow if a warm glass rod were rubbed with a piece of dry silk and then brought near the sealing-wax. On suspending, however, the electrified glass and bringing the electrified fur near it, repulsion would take place. It is manifest, then, that as electrified glass attracts electrified sealing-wax, but is repelled by B or repels electrified fur, there are two electrical states, one on the sealing-wax, which is called negative, and the other on the fur, which is called positive. It is also clear that bodies similarly electrified are mutually repellent, while bodies dissimilarly electrified are mutually attractive. This matter will again be referred to, but the points to which especial attention is now directed are, first, that the amount of positive electricity on the one body is always equal to the negative on the other; and, secondly, that the amount of electricity developed by rubbing the two bodies-say the sealing-wax and the fur---together, bears no direct relation to the amount of actual friction to which the bodies are subjected, for what is really essential in order to obtain the highest possible degree of electrification is to bring every portion of the one surface into intimate contact with every superficial particle of the other, and when that is done, no extra amount of rubbing can develop any further degree of electrification. Speaking generally, then, it may be said that when any two bodies are rubbed together electricity is produced, although it frequently happens that the amount is so small as to render its detection very difficult. If, however, delicate apparatus, which we will not pause here to describe, is employed, very feeble charges can be indicated. In fact, if a piece of zinc and a piece of copper are simply placed in contact the feeble charge of electricity then. developed can easily be rendered evident. If the same pieces of metal are dipped in saline or acidulated water, a similar result follows, although in this case the water itself becomes an important factor in determining the resultant electrification. The end of the zinc outside the liquid will be found to possess properties similar to those of the sealing-wax after it has been rubbed with fur. It is, therefore, said to be negatively electrified. The copper, on the other hand, will have an electrical state similar to that of the fur itself, or of the glass which has been rubbed with silk, and it is therefore said to be positively electrified. It follows that whether the electricity is the result of so-called friction or whether it is a consequence of the simple contact of two dissimilar bodies, it is with precisely the same kind of force that we have to deal, and the old distinction between 'frictional' and 'galvanic' electricity, which used to be urged with consider able persistence, is virtually a myth. The quantity of electricity may vary, and we may view it in its two phases, positive and negative, but neither of these considerations can affect the character of the force. It may be accepted as a general fact that when the same kind of force is bestowed upon two points or bodies, but to a different extent in the one case as compared with the other, there is a universal tendency to equalise the distribution of the force--that is to say, to produce equilibrium, and this equilibrium will be established when the conditions become such as to render it possible. Reverting again to the zinc and copper plates partly immersed in water, the exposed ends will be electrified to different degrees. There will be a tendency to produce equilibrium, or, as it is more generally called, neutralisation. This will be accomplished if the necessary facilities are afforded, and until this is done the intervening space will be subjected to what may be called an electrical stress. It is found by experiment that a piece of metal affords the readiest means of relieving the strain due to this stress, thus facilitating neutralisation, for on joining the two plates together, say by a piece of copper wire, a momentary rush of electricity from the one to the other will take place. This phenomenon is that which is generally known as discharge, and it affects the whole combination, including the liquid and the metal surfaces in contact with it. This brief spasmodic flow or rush of electricity, whose function it is to restore the electrical equilibrium, causes, however, a series of chemical changes to take place in the liquid itself, among other things a portion of the zinc being dissolved and converted into what is called a salt of that metal. These chemical reactions cause in their turn a fresh electrical difference between the plates, which is followed immediately by another equilibrating flow, and that by a further difference, and so on. These changes follow one another in exceedingly rapid succession, so rapid, in fact, that it is a matter of absolute impossibility to distinguish them separately, and we have consequently what appears to us as, and what is known as, a continuous 'current' of electricity. A little reflection will make it evident that by following out the line of experiment and deductions here indicated, the so |