currents are passed into the line simultaneously from both ends. In extending this principle of action, quadruplex telegraphy has been rendered possible, although not yet for long submarine lines.
The minute currents here employed are far surpassed as regards delicacy and frequency by those revealed to us by that marvel of the present day, the telephone. The electric currents caused by the vibrations of a diaphragm acted upon by the human voice naturally vary in frequency and intensity according to the number and degree of those vibrations, and each motor-current, in exciting the electro-magnet forming part of the receiving instrument, deflects the iron diaphragm occupying the position of an armature to a greater or smaller extent according to its strength. Savart found that the fundamental la springs from four hundred and forty complete vibrations in a second, but what must be the frequency and modulations of the motor-current and of magnetic variations necessary to convey to the ear, through the medium of a vibrating armature, such a complex of human voices and of musical instruments as constitutes an opera performance! And yet such performances could be distinctly heard and even enjoyed as an artistic treat by applying to the ears a pair of the double telephonic receivers at the Paris Electrical Exhibition, when connected with a pair of transmitting instruments in front of the foot-lights of the Grand Opera. In connection with the telephone, and with its equally remarkable adjunct the microphone, the names of Riess, Graham Bell, Edison, and Hughes will ever be remembered.
Considering the extreme delicacy of the currents working a telephone, it is obvious that those caused by induction from neighboring telegraphic line wires would seriously interfere with the former, and mar the speech or other sounds produced through their action. To avoid such interference the telephone-wires if suspended in the air require to be placed at some distance from telegraphic line wires, and to be supported by separate posts. Another way of neutralizing interference consists in twisting two separately insulated telephone-wires together, so as to form a strand, and in using the two conductors as a metallic circuit to the exclusion of the earth; the working current will, in that case, receive equal and opposite inductive influences, and will, therefore, remain unaffected by them. On the other hand, two insulated wires instead of one are required for working one set of instruments, and a serious increase in the cost of installation is thus caused. To avoid this, Mr. Jacob has lately suggested a plan of combining pairs of such metallic circuits again into separate working pairs, and these again with other working pairs, whereby the total number of telephones capable of being worked without interference is made to equal the total number of single wires employed. The working of telephones and telegraphs in metallic circuit has the further advantage that mutual volta induction between the outgoing and returning currents favors the transit, and neutralizes, on the other hand, the retard-