Knowing the cross section in millimeters, Q, of the bar, and observing
the temperature, t, in degrees Centigrade, its conductivity, x, as
compared with mercury can be determined. If L be the distance, h l or k
i, in meters, then
x = (1/m) (L/Q) (1 + at).
For pure metals the value of a may be taken at 0.004; but alloys have a
different coefficient. The instrument is made by Siemens and Halske,
and is accompanied by a table giving resistances per millimeter of the
measuring wire, N.--_Zeitsch. fuer Elektrotechnik_.
* * * * *
TERRESTRIAL MAGNETISM.
[Footnote: For a full account of experiments relating to magnetism on
railways in New York city, see SCIENTIFIC AMERICAN, January 19,1884.]
_To the Editor of the Scientific American_:
An item has appeared recently in several papers, stating that New York
is a highly magnetized city--that the elevated railroad, Brooklyn Bridge
cables, etc., are all highly magnetized. As this might convey to the
general reader the impression that the magnetism thus exhibited was
peculiar to New York city, and as many of your subscribers look
anxiously for your answers to numerous questions put for the elucidation
of apparent, scientific mysteries, I have thought that perhaps a
statement in plain language of experiments made at various times, to
elucidate this subject, might, in conjunction with a diagram, serve to
explain even to those who have not made a special study of science a few
of the interesting phenomena connected with
TERRESTRIAL MAGNETISM.
Pages:
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87