ES2 Dr. C. Chree on the Law 
§ 19. If we suppose the pole-distance in each magnet to be 
‘8 of its length, and neglect terms involving the radii of ‘the 
cross sections, except in the value of P, we obtain the 
following expressions for the couple between the two magnets 
(moments m and mj : 
Jones, or Cambridge | 
‘ Ino “|p? Op—4 —6§ 
Instrument.Company. j Qmm'r—?(1 +0:15r—“— 872r- *— 91807"), 
Coolie... ,.sdeswaceeen anne Qinm'r—3(1 +-7-96r—7 —5297r-4#— 131807), 
Dover ;,.:.shcvecaee Qma'r—3(1 +'7-08r—2 — 52874 — 118907), 
Fillioth. ; .;.ccsssneveeatens mmr —4( 1 4-7-78r—2 — 5349 *—18030r—), 
The relative importance of the different terms in the above 
formule is shown in Table XIII. for the typical Jones and 
Cooke magnetometers. 
TABiE cert, 
Jones Magnetometer. Cooke Magnetometer. 
'Deflexion| | 
distance |Principal P | Q KR |Principal) P Q R 
cms. term. | term. | term.| term. | term. | term, | term. | term. 
| + + ]/—- |---|] + + | -—- | = 
225 | 100000! 380 | 340 | 7 | 100000 | 1572 | 206 | 10 
| | 
96-25 | 100000 | 22 | 184 | 38 | 100000/ 1155) 111 | 4 
30 100000 17 | 108 | 1 | 100000} 884| 65 | 2 
35 | 100000) 12 | 38 0 | 100000} 650| 35 | 1 
' 40 |100000! 9 | 34] 0 | 100000] 497/ 21 | Oo 
) | 
| 
The results for the typical Elliott instrument are practically 
the same as for the Cooke; and those for the Dover instrument 
are practically the same in the case of the Q term, though 
some 10 per cent. less for the P and R terms. 
In the above typical case, p=0°8, the corrections (all +) 
TAREE ALY, 
j ! 
Jones or Camb. Inst. Co. . Cooke or Elliott. Dover. 
| | 
Distances ,),0H) dH (unit 17), i SH @H (unit ly). |, 956H|__ dH (unit 1y). | 
ems, 10° A H=185) H='37 10 H="185| H=37 |'0 | H=185) H=37 | 
225&30| 99 | 188 | 366 | 63 | 116 | 234 62 | 115 22-9 
sete) 53 9:8 196 | 33 61 122 | 33 61 12-2 
30&40 | 31 57 114 | 19 3:5 71 19 3°5 71 
