Page:Electricity (1912) Kapp.djvu/62

58 beam cannot dip either way, but it can be set into different angular positions by giving a twist to the upper end of the suspending wire, or if the upper end of the wire is held in a suitable clamp, it may set itself into an angular position in accordance with any electric force acting on the pith ball. The wire is clamped in a so-called torsion head at the upper end of a glass tube. By means of the torsion head any desirable amount of twist can be given to the upper end of the wire and read off on a circular scale, whilst the angular position assumed by the beam is indicated on a second scale placed at the level of the beam. To protect the apparatus from air-currents the beam is enclosed in a cylindrical glass vessel. In making experiments with static charges it is important to minimise as far as possible dissipation of the charges through the air, and for this reason precaution should be taken to keep the air dry. This is done by placing into the glass vessel a saucer with chlorate of potassium. Any moisture originally in the air is thereby extracted and rendered harmless. The cover of the vessel has an aperture through which is lowered a second gilded pith ball so as just to touch that on the scale beam.