MAGNETIC DISTURBANCE 1383 
It will be observed that no reading was obtained for the position 
directly above the casing. This was because of the fact that instrumental 
equipment was not available for measuring anomalies of this order. 
These data, with the exception of the last reading, are represented by 
the heavy curve of Figure 6, which illustrates the rapid rise of the mag- 
netic gradient in the immediate vicinity of the casing and the disappear- 
ance of the disturbance at 200 feet. Two significant facts bear particular 
emphasis at this point: (1) the magnetic distortion caused by this string 
of casing, which is more than 22 times the length of the previously men- 
tioned short pipe, disappears at very nearly the same radial distance; 
(2) there is no indication of a zone of negative anomalies as observed in 
the former case. 
COMPARISON OF THEORETICAL AND OBSERVED DATA 
For our theoretical treatment of the disturbing effect of magnetized 
casing we have based our discussion upon these fundamental conceptions: 
(x) the vertical string of casing may be regarded as a bar magnet having 
two definite magnetic poles; (2) the symmetry of the normal earth’s 
field suffers no distortion by virtue of the presence of the magnetized 
pipe; (3) the direction and magnitude of the composite magnetic field 
near the casing head is determined by the vector composition of the sec- 
ondary induced field and the normal terrestrial field. Though the 
validity of the last condition may be readily demonstrated, the two pre- 
ceding assumptions justify further elaboration. 
According to Jeans,' “The two ends of a magnet—or, more strictly, 
the two regions in which the magnetic properties are concentrated—are 
spoken of as the poles of the magnet.” Many investigators have devoted 
an imposing amount of theoretical discussion and research to the study 
of the distribution of magnetism in bar magnets of different dimensions, 
and it has been determined that under some conditions a regularly 
magnetized bar may be treated as having two definite points, one near 
each end, at which the positive and negative effects may be considered 
concentrated. However, in any case involving the assumption of 
magnetic distribution for the purpose of accurately interpreting the action 
of a magnet at an external point, it is required, as pointed out by Gray,’ 
that the point be removed a distance which is great in comparison with 
any dimension of the magnet. 
1J. H. Jeans, The Mathematical Theory of Electricity and Magnetism (Macmillan, 
New York), p. 364. 
2A. Gray, Absolute Measurements in Electricity and Magnetism (Macmillan, New 
York), p. 86. 
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