Elements of electricity, magnetism, and electro-magnetism : embracing the late discoveries and improvements : digested into the form of a treatise, being the second part of a course of natural philosophy : compiled for the use of the students of the University at Cambridge, New England / by John Farrar.

  • Farrar, John, 1779-1853.
Date:
1826
Catalogue details

Licence: Public Domain Mark

Credit: Elements of electricity, magnetism, and electro-magnetism : embracing the late discoveries and improvements : digested into the form of a treatise, being the second part of a course of natural philosophy : compiled for the use of the students of the University at Cambridge, New England / by John Farrar. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

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    181. We pass now to the examination of th? effects produced by the vertical components F, P. These may be easily calcu- lated by means of the dipping needle, which is shewn in figure 80, and of which we have already given some account. In this instrument the needle can only turn vertically round its horizon- tal axis, without going out of the vertical plane, where we place the circle in which it oscillates. Let us suppose, therefore, that this circle is directed perpendicularly to the magnetic meridian, Fig. 94. and then the horizontal forces H, H', which are parallel to this plane, will be completely destroyed by the pivots of the suspen- sion ; the vertical forces, F, P, therefore, acting alone, will tend to turn the magnetic axis in their own direction, that is to say^ to make it vertical, so that if we place it in this position, it will remain at rest. Here, then, we have a mark for recognising the direction perpendicular to the magnetic meridian ; we have only to turn the vertical circle, which carries the needle till this con- dition is fulfilled, and having noted on the horizontal division where it stops, to bring it back to 90° from this point, and it will be found in the magnetic meridian. In this new position, if we observe on the vertical circle the point of the division where the magnetic axis stops, the arc com- prehended between this division and the lower vertical point of the same circle will give the magnetic inclination, reckoned from the vertical of the place. In order that this observation and the preceding may be exact, great care must be taken to verify the horizontal position of the azimuth circle on which the instru- ment rests, and which for this purpose is furnished with two lev- els placed at right angles to each other. The dipping needle being directed in the magnetic meridian, and its magnetic axis being in equilibrium upon the direction of the forces which influence it, push it ever so litde out of the di- rection of its equilibrium, without making it deviate from the same vertical plane, it is manifest that the magnetic forces will tend to bring it back by a series of oscillations, the laws of which will be absolutely the same as those of a pendulum oscil- lating by the action of gravity. Without knowing the distribution of magnetism in each point of the needle, we conceive that the rapidity of its oscillations will <lepend upon this distribution, upon the absolute quantity of