=Paul’s Recorder.=—In the recorders previously described, the motive power is furnished by clockwork. R. W. Paul has introduced an instrument in which all the moving parts are actuated by a motor driven with power from the mains. This recorder is shown in fig. 25. The motor is furnished with a special type of governor to ensure constant speed, and is connected by suitable gearing to the mechanisms moving the chart, presser-bar, and inking ribbon, provision being made to vary the speeds of these movements by changing the gear. The galvanometer is of uni-pivot pattern, and the pointer is pressed at intervals on to a typewriter ribbon which lies above the chart. Immediately beneath the ribbon is placed a thin metal rod over which the paper passes, and the result of the contact is to produce a small dot. As in the thread recorder, the chart is divided into rectilinear coordinates, the ribbon in this case serving the same purpose as the thread in the former instrument. The lower part of the recorder is prolonged so as to display a considerable length of the chart, which is in the form of a roll, and is drawn forward by the mechanism. When two records are taken simultaneously the ribbon consists of two strips, one moistened with black ink, and the other with red; and it is arranged that each strip in turn is over the thin rod on to which the pointer is pressed, so that the records appear in separate colours. This recorder can also be arranged for multiple records, or fitted with a scale-control. With a view to workshop use, all the covers are fitted with faced metal joints, which are much better for keeping out dust than wooden ones. A further useful feature is that the various units in the recorder—galvanometer, motor, feed and record mechanism, and reducing gear—are all separate and interchangeable. By introducing a suitably divided chart this recorder will also serve for a radiation pyrometer, or, as will be shown later, for a resistance pyrometer. [Illustration: FIG. 26.—LEEDS-NORTHRUP RECORDER.]