electrical magnitude can be expressed. On a proper choice of units depends the simplicity of the expression for the laws which connect electrical phenomena. After describing these laws and measurements, the author has given their chief practical application to telegraphy and a few examples of the construction of telegraphic apparatus. These fluctuate in form from year to year, and the special forms now in use will soon become antiquated; but the general theory of Electricity on which the construction and use of these depends is permanent, depending on no hypothesis, and it has been the author's aim to state. this general theory in a connected manner and in such a simple form that it might be readily understood by practical men.

The above introduction is allowed to stand unaltered because it correctly describes what the author aimed at. He feels that the actual book falls very far short of the ideal he had conceived; he perceives only too well that the arrangement might be very greatly improved, and the statements made in much clearer language. The book has been unfortunately written in intervals snatched from professional engagements at irregular periods, but the author would rather claim indulgence on the score that the effort made has at least been in the right direction, although far from fully successful.

He has to acknowledge having received very kind assistance from his friends Sir W. Thomson, Professor J. C. Maxwell, Mr. Culley, and Mr. C. F. Varley; as well as from three of his assistants, Mr. W. Bottomley, Mr. W. E. Ayrton, and Mr. W. F. King, who kindly examined the proofs.

Mr. Latimer Clark and Mr. Culley have allowed free use to be made of extracts from their valuable handbooks.

CONTENTS.

CHAPTER I.

ELECTRIC QUANTITY.

§ 1. Definition of Electricity, and how it is produced by Friction ;

Conductors, Insulators. § 2. Resinous and Vitreous Electricity;

Attractions and Repulsions; meaning of a Charge.
§ 3.

Quantity of Electricity; depends on the measurement of Force.

§ 4. Experiments illustrating the foregoing; Electroscope. § 5.

Electricity at rest resides on the Surface of Conductors. § 6.

Justification of the names positive and negative Electricity.

§ 7. Attraction and Repulsion between Bodies positively and

negatively electrified. § 8. When Electricity is produced, equal

quantities of positive and negative Electricity are produced. § 9.

Electric Series or List determining the sign of the Electricity pro-

duced by Friction. § 10. Preliminary Explanation of the word

Potential. § II. Statical Induction. § 12. The existence of

any Charge implies an equal and opposite induced Charge.

§ 13. Induction implies two Conductors at different Potentials

separated by Insulators. § 14. Attractions and Repulsions con-

sidered as due to Induction. § 15. Distribution of Electricity

examined by Proof plane. § 16. Electrification does not imply

Charge at all points of Surface; Leyden Jar or Condenser.

§ 17. Meaning of the measurement of a quantity of Electricity.

§ 18. Absolute Electrometer measures Quantity. § 19. Pro-

duction of Electricity by other means than Friction; galvanic

Cell. § 20. Identity of Electricity, however produced. § 21.

Electricity produced by contact of Insulators. § 22. Electricity

produced by unequal distribution of Heat. § 23. Effect of a

Metal Screen between two electrified Bodies

CHAPTER II.

POTENTIAL.

PAGE

§ 1. Definition of Difference of Potentials. § 2. Work done in 26

moving Electricity from one Point to another is not affected by

Path followed. § 3. Constant Potential. § 4. The Potential

of a Body is the difference of its Potential from that of the

Earth. 5. On what electric Potential depends. § 6. Mean-

ing of higher and lower Potential. § 7. Illustration of

foregoing; Surface and Interior of electrified Conductor. § 8.

Space round charged Conductor. § 9. Illustration by Leyden

Jar. § 10. More complex Illustration. § 11. Effect of Changes

of electrification of Leyden Jar on Potentials of the several parts.

Effect of connecting two Jars. § 13. Relation between Charge

and Potential. § 14. Immaterial which coating of Leyden Jar

is to Earth. § 15. Theory of Electroscopes. § 16. Flow of

Electricity determined by difference of Potential. § 17. Effect

of joining a Conductor by a Wire with a Point of no Capacity

but of different Potential. § 18. Electricity in motion always

does work. § 19. Difference of Potential produced by Induction.

§ 20. Difference of Potential produced by Friction.
§ 21. Dif-

ference of Potential produced by Contact; Electric-contact Series

or List of Conductors. § 22. Analogies and differences in the re-

sult of contact in the cases of Solids and Liquids ; Galvanic-cell;

Electrolytes; Electrolysis. § 23. Electromotive Force, E. M. F.

§ 24. It is affected by Temperature. § 25. Currents of Elec-

tricity and Magnetism can produce E. M. F. § 26. Unit of

E. M. F. or difference of Potential.

52

CHAPTER III.

CURRENT.

§ 1 Definition of voltaic or galvanic Current. § 2. Transient and 52

permanent Currents. § 3. Currents involve the performance of

work. § 4. Is the Current due to contact or chemical action?

§ 5. Why no arrangement of Metals without Electrolytes can

give a Current. § 6. Attractions and Repulsions between Cur-

rents. § 7. Verification of Statements by Experiments; rectangle

and straight Wire. § 8. One Rectangle inside another. & a.