A beam compass is used for the purpose of drawing circles or ares of longer radius than can be taken in by the ordinary bow compass. A beam compass usually consists of a flat wooden beam fitted with two movable trammel heads such as are illustrated by Fig. 1. As is shown in the illustrations, these trammel heads carry interchangeable pen and pencil points, and dividers or needle points, and are secured to the beam by large clamping screws. The beam itself is usually a flat lath of hard wood, and when this lath is very long considerable inaccuracy may be caused by its deflection sideways. To prevent such deflection, the beam should be made of T section. In using the beam compass, the heads are adjusted approximately to the required distance, and are clamped in that position by the screws mentioned above; the exact distance is then adjusted by means of the fine adjusting screw lettered a in the illustration. Fig. 2 shows a beam compass, with a graduated beam, as used in the Ordnance Survey Department. To the fine adjustment is fitted a vernier scale, by which it is claimed that the distance between the heads can be regulated to the hundredth part of an inch. In Fig. 3 is shown a telescopic beam compass having several tubular parts sliding one within the other, and clamping screws to fix them at the desired position. A very neat and useful beam compass is that shown in Fig. 4, in which the beam is about 3/8 in. square. One of the heads is clamped to the beam, and serves to carry the pencil point, at the other end of which is the pen. The other head is held in position by the pressure of a strong spring, which presses a fluted roller against the top of the beam. A milled head at the side enables the draughtsman to rotate the fluted roller and so traverse the head along the beam to the desired position. A makeshift beam compass may be made out of a blind lath and two good-sized corks, such as are used in pickle bottles. Holes are burnt and cut for the reception of the lath and drawing pen as shown in Fig. 5, and also for the pricker or needle stuck into a penholder. The cork takes a good grip of the lath, and the instrument is quite steady and pleasant to work with.

Fig. 1.

Fig. 2.

Beam Compasses.