It will be observed of the two needles, R and R′, in the circuit of the same wire, C′, that if R is deflected to the right against the stop, V, then R′ will be deflected to the left against the metallic stop, S. The current, to produce these deflections, being through the wire C′, in the contrary direction to that indicated by the arrow of the wire, C′. But if R is deflected to the left against the metallic stop, T, then R′ will be deflected to the right against the stop, V. The current to produce these deflections, will then be through the wire, C′, in the direction of the arrow of that wire. The same effect is produced upon the two other pairs of needles of the wires, A′ and also B′. These contrary movements of the two needles, when a _current_ is passing, are produced by the coils being so wound, (see figure 69,) that the wire passes round one needle in a contrary direction to what it does round the other. If, now, we depress the keys, O and 7, the cups above and below, 5 and 50, and on each side of number 7, will be connected. The fluid will then pass from P or positive pole of the battery, 3′, to the lower cup, 50; then through the key wire to upper cup, 50; then along the extended wire, C′ to the _receiving_ station; then through the coils of the multipliers, deflecting the arm of the needle, R, to the left against the metallic stop, T; and the arm of the needle, R′, to the right against the stop, V, as indicated by the arrow at V; then to wire, 9 and D″; then along the extended wire back to the _transmitting_ station, to the right hand cup, 7; then by the key wire to the left hand cup, 7; then to wire, D; then to upper cup, 5; and through the key wire to lower cup, 5; then by the cross wire to upper cup, 60, and then to N, or negative pole of the battery. We have now shown the route of the current, when the keys, U and 7; and the keys, O and 7, were depressed. It will be observed, that when the keys, U and 7 were used, the current through the wire, D″, was from _left_ to _right_; and when the keys, O and 7, were used, the current was from _right_ to _left_. Thus, by means of the six keys, the current of each battery may be made to pass in either direction through the _common communicating_ wire, D″. By the keys, U, M, J, with 7, the current is made to pass from _left_ to _right_ along the wire, D″. By the keys, O, K, H, with 7, the current is made to pass from _right_ to _left_ along the wire, D″. By these six keys, all those various deflections of the six needles are produced, which are necessary to close the circuit of such of the wires, ̈1, ̈2, ̈3, ̈4, ̈5, ̈6, with the wire, 8, as are required for marking the signals desired, on an instrument now to be described. [Illustration: FIG. 71.] [Illustration: FIG. 72.]