The principle of this instrument, as already explained, is involved in the elementary experiment of Oersted--viz., the deflection of a magnetic needle from the inside of a coil of wire conveying a current of electricity, and as it is difficult to give a good description and drawing of the interior of the instrument that can really be understood, it may be sufficient to state that the handles give the operator the power of reversing the current of electricity, so that the needles are deflected with the utmost certainty to [Page 224] one side or the other, either separately or simultaneously. (Fig. 213.) [Illustration: Fig. 213. The letters of the alphabet, figures, and a variety of conventional signals, are indicated by the single and combined movements of the needles on the dial. The left-hand needle moving once to the left indicates the +, which is given at the end of a word. Twice in the same way, A; thrice, B; first right, then left, C; the reverse, D. Once direct to the right, E; twice, F; thrice, G. In the same order with the other needle for H, I, K, L, M, N, O, P. The signals below the centre of the dial are indicated by the parallel movements of both needles simultaneously. Both needles moving once to the left indicate R; twice, S; thrice, T. First right, then left with both, U; the reverse, V. Both moving once to the right, W; twice, X; thrice, Y. The figures are indicated in the same way as the letters nearest to which they are respectively placed. To change from letters to figures the operator gives H, followed by the +, which the recipient returns to signify that he understands. If, after the above signs (H and +) were given, C R H L were received, 1845 would be understood. A change from figures to letters is notified by giving I, followed by the +, which the recipient also returns. Each word is acknowledged. If the recipient understand, he gives E; if not, the +, in which case the word is repeated. Attention to a communication by this instrument is called by the ringing of a bell (of any size), which is effected through the agency of an electric current. The upper case contains the bell.]