300 EXPLORATION GEOPHYSICS 



The base of the instrument consists of two parts, placed one within 

 the other. The central section, which contains the clockwork mechanism, 

 is screwed to the base. This section has three leveling screws for setting 

 the instrument accurately in a vertical position. The lower part of the 

 central section contains a plug contact for supplying current to the contact 

 mechanism from a 4- to 6-volt battery, the latter being located under the 

 center of the torsion balance. An azimuth ring is provided which has 

 several adjustable stops for arresting the rotation of the instrument in 

 different azimuths. Stops are provided for three, four, and five symmet- 

 rical positions of the torsion balance. 



The contact mechanism is a precision clockwork which once an hour 

 closes a contact and lights the electric lamps at the top of the instrument for 

 making the photographic exposure. After the contact is opened, the upper 

 part of the instrument is rotated to the next stop by means of a mechanical 

 transmission gear between the contact clock and the driving mechanism. 

 The driving mechanism is mounted opposite the contact mechanism in 

 the central portion of the instrument. A gear of the driving mechanism 

 which meshes with the ring gear of the azimuth ring in the central portion 

 of the instrument causes the upper section to rotate quietly and steadily 

 until a lever in the driving mechanism runs against the next stop. The 

 upper section of the instrument can be set free to rotate by using a control 

 lever to disengage a friction clutch between the bevel pinion of the driving 

 mechanism and the ring gear. The clockwork and driving mechanism are 

 shielded from dust and damage by a cylindrical casing. The top plate of 

 the central section carries a plug-in contact for electrical connection be- 

 tween the contact clock and the top of the instrument. 



Current is supplied to the exposure lamps by a cable connected to the 

 battery. The brightness of each lamp may be adjusted separately by use 

 of rheostats. The light from the exposure lamps passes through a pinhole 



diaphragm, a prism, and then vertically down- 



p jj ^ ^ ^ ward to an adjustable mirror inclined at an 



.6 angle of about 45°; the beam of light is 



: i reflected horizontally from the inclined mirror, 



* passes through an achromatic lens to the mir- 



b ror on the stem of the balance beam, and is 



". s again reflected on to a photographic plate in 



the plate-holder attachment at the top of the 



Fig. 161.— Record taken with balance. The platc holder is mounted in a 



po^sitionf (r^'Sed 4^ timers) .*'' ^ slidc on rollers and can be moved across a slit 



F-fixed points, i.e.. spots of i" the basc of the plate-holder attachment at a 



light reflected from fixed speed of 3.5 mm. OCr hour. 



mirror. ^ -^ . . . . ^-, 



II— second balance beam. Light IS also reflected to the photographic 



I — first balance beam. , ° ii-,' i • r 



T— temperature points, i.e., plate irom two additional mirrors, one for 

 from ^mtrror^ attached'^' to eacli balance, which are mounted within the 

 Siritrip).'^^'''"^ ^^" innermost casing. One of these mirrors is 



