[47] The term “semi-conductor” is very rarely used by electricians, who prefer the term “partial conductor” as being more correct. Moreover, electricians, from Faraday downwards, are practically agreed in calling plumbago a good conductor, and worthy of being classified by reason of its high conductivity along with the metals. The substances known as “semi-conductors” are those given in Ferguson’s ‘Electricity,’ p. 49 (edition of 1873), namely, alcohol, ether, dry-wood, marble, paper, straw, and ice. Mascart and other eminent authorities agree in this classification. It would tax even Mr. Edison’s unrivalled ingenuity to make of these materials a transmitter that should alter its resistance by pressure! [48] Lüdtge’s German Patent, dated Jan. 12, 1878, describes a “Universal Telephone” in which a tympanum was applied to convey vibrations to an interruptor made of hard coke-carbon. APPENDIX III. COMPARISON OF REIS’S RECEIVERS WITH RECENT INSTRUMENTS. The receivers invented by Reis for the purpose of reconverting into audible mechanical vibrations the varying electric currents transmitted from the speaking end of the line were of two classes, viz.: (1.) Those in which the magnetic expansion and contraction of a rod of steel or iron, under the influence of the varying current, set up mechanical vibrations and communicated them to a sound-board. (2.) Those in which the current by passing round the coils of an electro-magnet caused the latter to vary the force with which it attracted its armature, and threw the latter into corresponding mechanical vibrations.