For the sake of clearness, the diagram has been drawn with simple lines only. In the real needle-machine the construction is much more complicated; perspective drawings of it may be seen in Lardner’s “Electric Telegraph,” and numerous other works. In fig. 1, B is a single cell of a battery containing a plate of copper, C, and a plate of zinc, Z, immersed in sulphuric acid and water. H is the handle of the instrument, turning from left to right, and _vice versâ_, like the handle of a door, consisting of two pieces of brass insulated from each other by being inserted in an axis of ivory. To the ends of the two pieces of brass are fixed the wires, CW and ZW, leading from the copper and zinc ends of the cell respectively. Fixed on each side of the handle are two plates of metal, which may be called LP, the left plate, and RP, the right plate. They are connected with the needle wire, NW, which passes before and behind the magnetized needle, N, suspended perpendicularly on its axis, with its north pole upwards. As long as the wires, CW and ZW, remain insulated from each other, no current passes from the cell; but as soon as the handle, H, is turned, so that the copper end touches the left plate (fig. 2), and the zinc end touches the right, communication is established between the plates of the cell, and the current commencing at the copper passes along CW, the top half of H, into LP, along N W, travelling _up before_ and _down behind_ the needle, causing it to deflect to the observer’s left, according to the rule given above. Reaching RP, it passes downwards to the cell to Z, and so on to C, continuing its travels and keeping the needle deflected as long as the handle remains in contact with the plates. If it is required to deflect the needle to the right, the handle is turned to the right, bringing its copper end in contact with the right plate, causing the current to travel in the opposite direction. By following the current from the copper to the zinc, as indicated by the arrows in fig. 3, it will be seen that it now travels _up behind_ and _down before_ the needle, deflecting it to the observer’s right. Thus, by causing a current of voltaic electricity to pass alternately _up before_ and _down behind_, and _up behind_ and _down before_, the needle is moved to the left or the right at will. The way in which the current is made to act on a distant needle is now simple. The following figure (fig. 253) shows the arrangement. The left portion of the figure represents an instrument at London, that on the right an instrument at York. The needle-wire, instead of being continued directly to the zinc plate of the battery, passes away from the needle over poles to York, where it joins an instrument similar in all respects to that at London. It passes similarly before and behind a magnetized needle, joining the right plate of the instrument. As long as the two plates are unconnected, no current can pass. The current is