ERRATA.

Page 31, line 9 from bottom, before "elastic" insert "stretched."

60, line 7 from bottom, for "270" read “2wo/K."

62, line 17 from bottom, for "expression" read "expressions."

80, line 21, for "and consists" read "and usually consists."

84, line 1, for “ Lichtemberg" read “Lichtenberg.”

122, line 5, for "hydroscopic" read “hygroscopic.”

,, 124, line 3 of § 90, for "represents " read "represent."

156, line 12 from bottom, after "as" insert "numerically equal to."

156, line 9 from bottom, for “Where” read “When."

157, line 7, at the beginning, insert "numerically equal to."

220, at the middle, for “Cu + SO4” read “Cu + ZnSO4."

262, line 15 from bottom, at the beginning, insert “. per cm3."

262, line 4 from bottom, at the end, insert "per cm3."

Page 289, lines 11 and 12 from bottom, for "2

MM'

R3

(1 +ƒ)" read

(1+f), where ƒ is a constant depending on the

303, line 19, for "infinite" read "infinitely long."

335, last line, for "force" read "induction."

336, line 11, for "Kirchoff" read "Kirchhoff."

339, line 18 from bottom, for "infinite" read "infinitely long."

348, second line of § 292, for "phenomenon" read "pheno

mena."

360, line 1, before “resulting" insert "quantity of the."

360, line 13, before "current" insert "quantity of the."

435, line 11, in numerator of the second equation, for "E"

436, line 10 from bottom, before "in order" insert "since.” ,,436, line 7 from bottom, for "measured; or " read "measured,"

a

2

507, line 5, for "carbon, held in a suitable handle," read
"carbon rod connected with the positive pole."

ELECTRICITY.

CHAPTER I.

FUNDAMENTAL PHENOMENA.

1. Electrification by Friction.-If a plate of glass and a plate of vulcanised india-rubber, both thoroughly dry, are laid on each other and rubbed together, it is found that some force is required to separate them, and that, after having been separated, they attract each other if not too far apart. If a second piece of glass and a second piece of india-rubber are treated in the same way, it is found that not only does either piece of glass attract either piece of india-rubber, but that the two pieces of glass repel each other, and that the two pieces of india-rubber do the same. Bodies which are in such a condition as to exhibit these properties are said to be electrified.

Attraction also occurs when an electrified body is brought near bodies that have not been subjected to friction, and if these are light enough (bits of pith, feathers, shreds of paper, wool, &c.), they may be lifted. Effects of this kind often afford an easier test of the existence of electrification than such experiments as those previously referred to. It is accordingly to be noted that although attraction occurring between two bodies may be evidence of previous electrification, it does not prove that both the mutually attracting bodies were previously electrified; and if only one was thus electrified, we cannot say, without other evidence, which it was. On the other hand, repulsion, as between two pieces of glass or two pieces of india-rubber, does not occur unless both the repelling bodies have been previously electrified.

2. Electrification by Contact.-If a pith-ball (A) hung by a fine

A

fibre of raw silk (Fig. 1) is allowed to touch a piece of glass (D) that has been electrified by friction, it is repelled by the glass after contact (Fig. 2). Again, a second pith-ball, similarly hung by a

silk fibre, if allowed to touch a piece of electrified india-rubber, is repelled by the india-rubber after contact. The two pith-balls, however, if brought near together, are found to attract each other; in fact, they respectively behave towards the glass and india-rubber,

TT

FIG. 3.

and towards each other, just as a second pair of pieces of glass and india-rubber which had been rubbed together would do. That is, the two pith-balls have acquired properties exactly like

those of the glass and india-rubber, and are consequently said to be electrified by contact.

A metal rod or cylinder supported by a dry glass stem, or suspended by dry silk threads, can be electrified by contact with a piece of rubbed glass or of rubbed india-rubber. A pith-ball hanging by a silk fibre so as just to touch the metal before electrification may be repelled in this way to a considerable distance (Fig. 3).

A piece of metal thus electrified exhibits signs of electrification at all parts of its surface, but the evidence of electrification is less marked the larger the piece of metal. At the same time, the electrification of the primarily electrified body is found to be lessened by having imparted its own properties to the metal. In fact, the whole process takes place as though the communication of electrification depended on the sharing of some thing or condition, existing at the surface of the originally electrified body, between it and the body with which it is brought into contact, so that what is gained by one is lost by the other. Facts of this kind have led to the idea of an unknown something capable of being developed on bodies by friction, when two different substances are rubbed together, capable also of spreading from one body to another by contact, and giving rise to the phenomena characteristic of electrification. This supposed cause of electrical phenomena is spoken of as electricity.

3. Good and Bad Electrical Conductors.- Bodies which, when electrified at one part, are immediately found to be electrified all over, are called good conductors of electricity, and the process is commonly described as depending on the freedom with which electricity can move over them. There are, however, many substances—such as glass, shellac, india-rubber, silk, wool, sulphur, amber-which, when electrified at one part, allow the electrical state to spread only very slowly and gradually to other parts. Such bodies are conceived of as not permitting the passage of electricity through them or along their surface: they are hence called non-conductors of electricity. More properly, bodies of this kind are called bad or slow conductors, for they do not absolutely prevent the communication of electrification, and hence their properties differ rather in degree than in kind from those of good conductors. The differences in degree, however, are often enormous, the spread of electrification being in some cases apparently instantaneous, while in