667315-1886-Verification-of-the-Unifilar-Magtometer-Elliott-Bro--No-55- — Page 3

468

THE HONGKONG GOVERNMENT GAZETTE, 29TH MAY, 1886.

H

Value of 1 +

F

TABLE III.

for different values of the deflection produced by a twist of 90° of the suspending thread.

Effect of 90° torsion.

H

1+

F

0.25 div.

1.00008

0.50

1.00017

0.75

1.00025

1.00

1.00034

1.25

1.00043

1.50

1,00052

1.75

1.00060

2.00

1.00068

2.25

1.00077

2.50

1.00086

2.75

1.00094

3.00

1.00103

TABLE IV.

Reduction to 0°

t

qt + q' t2

t

qt + q' t2

+ 14°

+.00412

+ 27°

+.00881

15

.00446

28

00920

16

.00480

29

.00960

17

.00514

30

.01001

18

.00549

31

.01042

19

:00584

32

.01084

20

.00619

33

.01126

21

.00655

34

.01168

22

.00692

35

.01211

23

.00729

36

.01254

24

.00766

37

.01297

25

.00804

38

.01341

26

.00842

39

.01385

+ 27

.00881

+ 40

.01430

TABLE V.

Value of logr3 at different temperatures.

t

r = 30

ro -

40

.00

4.13033

4.50515

4.13045

4.50527

10

4.13057

4.50538

15

4.13069

4.50550

20

4.13080

4.50562

25

4.13092

4.50574

30

4.13104

4.50585

35

4.13116

4.50597

40

4.13127

4.50609

On the determination of the declination.

The zero or true north point on the horizontal circle of the magnetometer is ascertained as follows from observations of Polaris. The horizontality of the axis of the speculum having been verified th speculum is placed vertically and revolved round its vertical axis till the image of the wires, a high are illuminated by a lamp held beside the collimating eye-piece of the telescope, appears in the held. The speculum is turned by its slow motion screw till the image of the vertical wire is covered by the latter itself. The instrument is then revolved round its vertical and the speculum round its horisontal axis till the image of Polaris is bisected by the wires, and both verniers of the circle are then read. This operation is repeated after reversing the mirror on its pivots. The speculum is then turned half a circumference round its vertical axis and the operation repeated directly and again after reversing the mirror on its pivots. The image of the vertical wire is therefore four times covered by the wire.

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