Cathetometer

Cathetometer is an instrument employed in physical research to measure accurately the distance between two points in a vertical line. Thus the volume of gas enclosed in a vertical graduated tube may be read at a distance from the tube by means of a cathetometer. In such a case it may be undesirable to approach closely to the tube, as the heat of the body of the observer may alter the volume required to be measured. The instrument is essentially a horizontal astronomical telescope capable of sliding up and down a graduated vertical scale. This telescope is first placed so that its axis points to one end of the required length, two fine spider-lines fitted at right angles to each other in the telescope helping to effect this adjustment accurately. The position of the telescope is read off on the vertical scale of the instrument and the telescope is then moved up or down till its axis points to the other end of the required length. Then its new position is again read off. The distance traversed by the telescope is the distance between the two ends of the required length, which is therefore determined.