The velocity acquired in falling for one second in vacuo, in any part of Great Britain, is about 32.2 feet per second, or 9-81 metres per second. The pressure of one atmosphere, or 760 millimetres (29.922 inches) of mercury, is 1033 kilogramme per sq. centimetre, or 14.73 lbs. per square inch. The weight of a litre of dry air, at this pressure (at Paris) and 0° C., is 1·293 gramme. The weight of a cubic foot of water is about 62.4 lbs. 772 for the foot and Fahrenheit degree. 1390 for the foot and Centigrade degree. is 42400 for the centimetre and Centigrade degree. In absolute units of energy, the equivalent is 41.6 millions for the centimetre and Centigrade degree; 1 gramme-degree is equivalent to 41.6 million ergs. UNITS EMPLOYED BY PRACTICAL ELECTRICIANS. The ohm is (or was intended to be) 10° C. G. S. electro-magnetic units of resistance. The volt is 10° C. G. S. electro-magnetic units of electro-motive force. The ampère is of the C. G. S. electro-magnetic unit of current, and is the current produced by an electro-motive force of 1 volt in a circuit whose resistance is 1 ohm. The coulomb is of the C. G. S. electro-magnetic unit of quantity, and is the quantity conveyed in 1 second by a current of 1 ampère. The farad is 10-9 of the C. G. S. electro-magnetic unit of capacity, and the microfarad is the millionth part of the farad. A charge of 1 coulomb given to a condenser of capacity 1 farad would raise its potential by 1 volt. The watt is 107 ergs per second, and is the rate at which work is done by 1 ampère traversing 1 ohm. The joule is 107 ergs. ELECTRICITY. CHAPTER XLI. INTRODUCTORY PHENOMENA. 556. Fundamental Phenomena.-If a glass tube be rubbed with a silk handkerchief, both tube and rubber being very dry, the tube will be found to have acquired the property of attracting light bodies. If the part rubbed be held near to small scraps of paper, pieces of Fig. 332.-Attraction of Light Bodies by an Electrified Body. cut straw, sawdust, &c., these objects will move to the tube; sometimes they remain in contact with it, sometimes they are alternately attracted and repelled, the intensity as well as the duration of these effects varying according to the amount of friction to which the tube has been subjected. If the tube be brought near the face, the result is a sensation similar to that produced by the contact of a cobweb. If the knuckle be held near the tube, a peculiar crackling noise is heard, and a bright spark passes between the tube and knuckle. The tube then has acquired peculiar properties by the application of friction. It is said to be electrified, and the name of electricity is given to the agent to which the various phenomena just described are attributed. Glass is not the only substance which can be electrified by friction; the same property is possessed also by resin, sulphur, precious stones, amber, &c. The Greek name of this last substance (λEKTрov) is the root from which the word electricity is derived. At first sight it appears that this property of becoming electrified by friction is not common to all bodies; for if a bar of metal be held in the hand and rubbed with wool, it does not acquire the properties Fig. 333.-Electrification of a Metal by Friction of an electrified body. But we should be wrong in concluding that metals cannot be electrified by friction; for if the bar be fitted on to a glass rod, and, while held by this handle, be struck with flannel or catskin, it may be very sensibly electrified. There is therefore no basis for the distinction formerly made between electrics and nonelectrics, that is, between substances capable and incapable of being electrified by friction; for all bodies, as far as at present known, are capable of being thus excited. There is, however, an important difference of another kind between them, which was first pointed out by Stephen Grey in 1729. 557. Conductors and Non-conductors.-In certain bodies, such as glass and resin, electricity does not spread itself beyond the parts of the surface where it has been developed; while in other bodies, such as metals, the electricity developed at any point immediately spreads itself over the whole body. Thus, in the last-mentioned experiment, the signs of electricity are immediately manifested at the end of the metal bar which is farthest from the glass rod, if the end next the rod be submitted to friction. Bodies of the former kind, such as glass, resin, &c., are said to be non-conductors. Metals are said to be good conductors. A non-conductor is often called an insulator, and a conductor supported by a non-conductor is said to be insulated. The appropriateness of these expressions is evident. No substance is perfectly non-conducting, but the difference in conduct CONDUCTORS AND NON-CONDUCTORS. 547 ing power between what are called non-conductors and good conductors, is enormous. The following are lists of conductors and non-conductors, arranged, at least approximately, in order of their conducting powers. In the list of conductors, the best conductors are put first; in the list of non-conductors, the worst conductors (or best insulators) are put first. The human body is a good conductor of electricity. If a person standing on a stool with glass legs be struck with a catskin, he becomes electrified in a very perceptible degree, and sparks may drawn from any part of his body. be When an insulated and electrified conductor is allowed to touch another conductor insulated but not electrified, it is observed that, after the contact, both bodies possess electrical properties, electricity having been communicated to the second body at the expense of the first. If the second body be much the larger of the two, the electricity of the first is greatly diminished, and may become quite insensible. This explains the disappearance of electricity when a body is put in connection with the earth, which, together with most of the objects on its surface, may be regarded as constituting one enormous conductor. On account of its practically inexhaustible capacity for furnishing or absorbing electricity, the earth is often called the common reservoir. It will now be easily understood why it is not possible to electrify a metal rod by rubbing it while it is held in the hand; since the |