Elements of electricity, magnetism, and electro-magnetism : embracing the late discoveries and improvements : digested into the form of a treatise, being the second part of a course of natural philosophy : compiled for the use of the students of the University at Cambridge, New England / by John Farrar.

  • Farrar, John, 1779-1853.
Date:
1826
Catalogue details

Licence: Public Domain Mark

Credit: Elements of electricity, magnetism, and electro-magnetism : embracing the late discoveries and improvements : digested into the form of a treatise, being the second part of a course of natural philosophy : compiled for the use of the students of the University at Cambridge, New England / by John Farrar. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

    18/428 (page 4)
    The atmosphere is evidentlj'- of the class of non-conducting bodies; since, if it afforded a free passage to electricity, no body surrounded by it could exhibit durable electrical phenom- ena. Now a tube of glass or resin, being rubbed, preserves its electric properties for a considerable time, although immersed in this fluid. Water, on the other hand, is a conductor ; for if we moisten with this liquid or only with its vapour, a tube of glass or resin, electrified by fi'iction, it immediately loses all its virtue. Thus the aqueous vapour suspended in the air impairs the insulating properties of this fluid, and it is for this reason that electrical experiments succeed best in cold and dry weather, because then there is less vapour contained in the atmosphere. This diflference in the disposition of different bodies to retain and transmit electricity, was first made known by Grey. He owed the discovery to accident, but to an accident of which he well knew how to avail himself. There is no constant relation between the state of bodies and their conducting power. Among solid bodies, the metals trans- mit electricity readily, dry gums and resins scarcely transmit it at all. Almost all liquids are good conductors ; oil, however, is a very imperfect conductor. Wax and tallow, when cold, con- duct badly ; when melted, they conduct well. The power of conducting electricity is observed in the most opposite states; for example, in the flame of alcohol and in ice. The tempera- ture of bodies seems to have no sensible influence on the elec- tric sparks which proceed from them. Those which proceed from ice are not coid, and those which proceed from red hot iron, do not appear to have their heat increased. The air and dry gases, besides their insulating property, seem also to have the faculty of confining electricity upon the surface of bodies by the force of pressure. For, if we place under the receiver of an air-pump, a conducting body electrified and insulated upon supporters of glass or resin, this body, when lectrics, or non-electrics, because it was believed that the first only could be electrified by friction. This was an error. All bodies become eleclrical hy being rubbed, but all are not capable of retaining the electricity thus developed, without being insulated.