Chap. 7] GRAVITATIONAL METHODS 125 



It is seen that for high (mechanical) sensitivity an increase in period 

 (astatization) is of advantage. However, not all gravimeters are astatized. 



The Threlfall and Pollock gravimeter^^ is one of the earliest examples of 

 an unastatizcd gravimeter. It consists of a torsion wire about 0.0015 in. 

 in diameter, supporting in the middle a quartz bar about 5 cm long and 

 0.018 g hi weight, whose position is read by a microscope. One of the 

 studs holding the wire is fixed ; the other is rotated until the bar end coin- 

 cides with the crossweb in the microscope (horizontal position). The 

 corresponding stud position is read on an accurate dial. 



The Wright gravimeter closely resembles the Threlfall-Pollock instru- 

 ment, two tapering helical springs taking the place of the torsion wire. 

 Between them a small boom with a mirror is adjusted to horizontal position 

 by turning one of the studs supporting the springs. An illustration based 

 on a patent drawing is given in the author's publication on gravimeters.^^ 

 In the most recent type " measurements are made as follows: The springs 

 arc wound until the boom is horizontal, at which time the reading of the 

 spring-supporting frame is recorded. Then the spring is unwound so that 

 the boom passes through its vertical position and reaches a horizontal 

 position on the other side. The corresponding reading is recorded; the 

 difference between readings for the horizontal boom positions is a measure 

 of gravity. Since the boom is used near the upsetting position, this 

 instrument may be included in the group of astatic gravimeters. Accurate 

 temperature and pressure control is required; the accuracy is about 1 

 miUigal. 



The Lindblad-Malmquist ("Boliden") gravimeter*^ consists of two 

 springs carrying a light mass with two disk-shaped extensions (see Fig. 

 7-28a). The upper disks act as the variable condenser in an "ultramicrom- 

 eter" circuit. Using a spacing of. 2-10~' cm, the authors claim to have 

 been able to detect displacements of the order of 3.5-10"^ cm. A gravity 

 change of one milligal corresponded to a displacement of about 5.5 -10"^ 

 cm, so that variations of the order of 1/100 miUigal would be detectable. 

 Because of various interfering factors, however, the mean error in the field 

 was 0.1 to 0.2 milligal for a single observation and 0.05 to 0.1 milligal for 

 five to ten observations. The ultramicrometer circuit acts merely as an 

 indicator, the deflections being compensated by electrostatic attraction 

 between the upper plates. The distance between the lower plates is so 

 adjusted that a potential difference of 10 volts corresponds to a gravity 

 variation of 1 milligal. 



38 Phil. Trans. Roy. Soc. (A) 193, 215-258 (1900); (A) 231, 55-73 (1932). 

 " C. A. Heiland, A.I.M.E. Tech. Publ., 1049 (1939). 

 ^o F. E. Wright and J. L. England, Am. J. Sci., 35A, 373-383 (1938). 

 *i A. Lindblad and D. Malmquist, Ingen. Vetensk. Handl., No. 146, 52 pd. (Stock- 

 holm, 1938). 



