Lectures on electricity, comprising galvanism, magnetism, electro-magnetism, magneto- and thermo-electricity / By Henry M. Noad ; illustrated by nearly three hundred wood cuts.

  • Henry Minchin Noad
Date:
1844
Catalogue details

Licence: Public Domain Mark

Credit: Lectures on electricity, comprising galvanism, magnetism, electro-magnetism, magneto- and thermo-electricity / By Henry M. Noad ; illustrated by nearly three hundred wood cuts. Source: Wellcome Collection.

    22/482 page 8
    9°. That silk is a substance which possesses this power of in¬ sulating. 10°. That metallic bodies and thread soaked in salt and water, do not possess this power. (18) But certain other effects attend the excitation of glass or resin: if either be briskly rubbed while dry and warm, in a darkened room, light will be perceived ; a slight crackling noise will be heard; and if the hand be held near, a sensation similar to that which is felt when we touch a cobweb, will be experienced. (19) The difference which in the previous experiments we perceived between bodies such as silk and glass, and bodies such as cotton, thread, and metal, arises from the former conducting Electricity very badly, while the latter offer a ready passage to the same. On this account bodies have been divided into two great groups—-conductors and non¬ conductors ; the former being in general identical with analectrics, and the latter with ideo-electrics. But it must be observed that the line of demarcation between these two great classes is by no means strictly de¬ finable ; as a large number of substances exist which conduct Elec¬ tricity when present in large quantities, but insulate it when in small. (20) Among conducting substances may be classed all metals, water, steam, all animal and vegetable substances containing water, &c.; while glass and other vitrifications, gems, resins, sulphur, metallic oxides, organic substances perfectly free from water, and ice, are all more or less perfect non-conductors, or ideo-electrics. A substance placed upon a non-conductor, as on a stool with glass legs, is said to be insulated from the earth. Atmospheric air must, it is clear, be ranked among non¬ conducting bodies, for if it gave a free passage to Electricity, the elec¬ trical effects excited on the surface of any body surrounded with it would quickly disappear, and no permanent charge could be commu¬ nicated ; but this is contrary to experience. Water, on the other hand, whether in the liquid or vaporous form, is a conductor, though of an order very inferior to that of the metals; and as water is always present in the atmosphere, it affects, in a very important manner, all electrical experiments. Hence, one of the reasons why these are made with more facility, and the desired effects produced with more certainty and success in cold and dry weather, the atmosphere then holding but little aqueous vapour suspended in it. Another injurious tendency of the watery vapour in the atmosphere is that which it has to become deposited on the surfaces of bodies, thereby destroying their insulating power;—hence the non-conducting power of the glass or resinous sup¬ ports of electrical apparatus can only be preserved by constantly rubbing or wiping them with a dry warm cloth, or silk handkerchief:
    digitised image 1
    digitised image 2
    digitised image 3