A drum d (Fig. 28) carries a strip of paper wound upon it. The surface has grooves cut in it, either circular or (if to indicate for more than 12 hours without attention) spiral. In the latter case, one end of axis of drum is cut with a screw of the same pitch as the grooves on the drum, and works in a fixed nut, so that, when rotated, the drum travels along an axial line at such a rate that the grooves retain the same relative position to a fixed point, such as the indenting point, presently to be described. The drum has the hours engraved at one end, and is made to rotate once in 12 hours. A spring (not shown) holds the cylindrical armature off the face of electro-magnet m until a current passes, when the lever l is drawn down, and the point p makes a puncture in the paper. The position of this puncture relatively to lines drawn from the figures on end of drum along the paper in direction of drum's length, will indicate the time at which current passes. A simple contact spring, such as is used for ringing electric bells, is pushed by the watchman when making his rounds, and a record is made on the paper.

Fig. 28.

Tell Tale Clock 30030