PREFACE. THE Appendix to a small work on Electrical Measurements," by one of the Authors, containing many electrical data and formulæ, having proved useful, and met with approval amongst telegraphists, they have been induced to undertake the following more complete compilation, which they believe will supply an admitted want. In bringing together such a heterogeneous mass of materials it has been found difficult to follow consistently any systematic plan of arrangement; but it is hoped that a tolerably copious index will render this unavoidable absence of system a matter of small importance. In the following pages, the "specific resistance" of any insulator has been assumed to be the resistance of a cube knot of the material, at 75° F., calculated from its measured resistance in the form of a cable. In the same way, its "specific electro-static capacity" is taken as the * "An Elementary Treatise on Electrical Measurements," &c., by Latimer Clark. E. & F. N. Spon, Charing Cross, 1868. |