In the Duddell Thermo-galvanometers, therefore, the alternating current is first led to a "heater" consisting of fine platinised quartz fibre or thin metal wires. Just above the heater there hangs a thermo-couple, consisting of two little bars, one of bismuth and the other of antimony. These two are connected together at their lower end, where they nearly touch the heater, but their upper ends are kept a little apart, being joined, however, by a loop formed of silver strip. This arrangement will be quite clear from the accompanying sketch, and it will be observed that the loop is so shaped that the whole thing can be easily suspended by a delicate fibre which will permit it to swing easily, like the coil in a mirror galvanometer. It is indeed a swinging coil of a galvanometer formed with a single turn instead of the many turns usual in the ordinary instruments, and it will be noticed from the sketch that there is a mirror fixed just above the top of the loop. This coil, then, with the thermo-couple at its lower extremity, is hung between the ends of a powerful magnet much as the fibre of the Einthoven Galvanometer is situated. The alternating current to be measured comes along through the heater. The heater rises in temperature. That warms the lower end of the thermo-couple. Instantly a steady, continuous current begins to circulate round the silver strip which forms the coil, and that, acting just as the current does in the ordinary galvanometer, causes the coil to swing round more or less, which movement is indicated by the spot of light from the mirror. A current as small as twenty micro-amperes (or twenty millionths of an ampere) can be measured in this way. Mr Duddell has also perfected a wonderful instrument called an Oscillograph, for the strange purpose of making actual pictures of the rise and fall in volume of current in alternating circuits. [Illustration: FIG. 3.--The "Duddell" Thermo-galvanometer. In this remarkable instrument _alternating_ current enters at _a_, passes through the fine wire and leaves at _b_. In doing this it heats the wire, which in turn heats the lower end of the bismuth and antimony bars. This generates _continuous_ current, which circulates through the loop of silver wire, _c_, which, since it hangs between the poles, _d_ and _e_, of a magnet, is thereby turned more or less. The amount of the turning indicates the strength of the _alternating_ current.]