Reductions to Standard Instruments 15 



described. As this instrument is intended entirely for land use the mechanism for the 

 rotation of the coil was modified in such a -way that the cup-shaped gear is rotated directly 

 by a crank in the axle of the ring carrying the inductor coil. The galvanometer used with 

 this instrument is of the Kelvin type as manufactured by the Leeds & Northrup Company, 

 Philadelphia, Pa. (see Plate 4, Fig. 4). The coil-frame supports are mounted above the 

 base by corrugated rubber pillars; brass tubes telescope over these pillars to protect them 

 from dust when not in use. The four coils are each of about 31 ohms resistance and so 

 arranged that they may easily be connected in series or multiple. The control magnets 

 originally supplied by the manufacturer were discarded because they were of such size as 

 to affect the results obtained for inclination when the galvanometer was mounted close 

 enough to the instrument to permit one observer both to rotate the earth-inductor coil 

 and to read the galvanometer; accordingly control magnets of smaller size were substituted. 

 The galvanometer may thus be mounted within 3 feet of the earth inductor. This appa- 

 ratus has been loaned to the Royal Alfred Observatory at Mauritius. 



In using the earth-inductor attachment of the universal magnetometer it is essential 

 that the tripods be mounted on pegs driven well into the ground. The screw adjustment 

 of the commutator of the inductor should be such as to give a light, firm contact without 

 too much friction, and the conmiutator should be kept well lubricated. The commutator 

 end of the coil is always to be down, as otherwise the attachment for rotating the coil would 

 interfere at times with the base of the instrument. In using the flexible shaft the coil is 

 brought up gradually to full speed and stopped gradually (by careful handling the flexible 

 shaft can be kept in good condition and any tendency towards kinking avoided). 

 Particular care must be used to see that there are no loose connections either at the gal- 

 vanometer or at the inductor, as these sometimes cause trouble. The inductor coil must 

 always be carefully examined before making connections to make sure that there is no 

 looseness in its bearings; any looseness is taken up by means of the milled nuts at the end 

 of the bearing where the flexible shaft is attached, care being taken, however, to see that 

 the bearings are not too tight, and particularly to see that they are clean and free from 

 grit, dust, or sand. With these instruments it is particularly necessary that great care 

 be given the level adjustment. 



A list of the various circles and earth inductors used, together with the needles and 

 their designations, will be found in Table 3, "Dip Corrections on Adopted C. I. W. 

 Standard for the Period 1911-1913," pp. 17-20. 



REDUCTIONS TO STANDARD INSTRUMENTS. 



The world-wide operations of the Department of Terrestrial Magnetism have 

 necessitated extensive intercomparisons of magnetic instruments at Washington as 

 well as in the field and at magnetic observatories in the regions covered; the results 

 of these intercomparisons to 1914 will be found on pages 211 et seq. With thedata thus 

 obtained it has been possible to refer the magnetic elements for the entire region 

 embraced in Volume I and in this publication to magnetic standards within an 

 error, in general, on the order of the error of observation. Wliile the adopted 

 standards are provisional, the numerous comparisons with magnetic observatory 

 standards show that they approach so close to international ones that the correc- 

 tions adopted may be considered as fulfilling all practical requirements of a general 

 magnetic survey of the Earth. 



