A direct recording declinograph

Abstract

A preliminary paper on this instrument was published in this journal for June, 1904. (Vol. IX, p. 69.) Under the auspices of the Department of Terrestrial Magnetism of the Carnegie Institution two portable declinographs have since then been constructed under the writer's direction, one of which is at present installed at the Solar Observatory on Mt. Wilson, California. As these instruments embody several features not contained in the original experimental apparatus, a short description of them is here given.It will be remembered that the principle of the instrument consists in the transmission of the movements of a suspended magnetic needle, through a delicate suspended glass lever to a pen situated at the end of a long arm. The lever system is therefore somewhat similar to that in the Bosch‐Omori seismograph, except that no pivots are used. The directive force acting on a magnetic needle in the Earth's field, is so much feebler than that in the case of a massive horizontal pendulum, that it was thought best to avoid the slight but uncertain friction of pivots. The pen is caused periodically to print a dot upon a long strip of paper, which is advanced by means of clockwork. The most radical departure from the original apparatus is in the manner of printing dots: the pen is no longer drawn down to the paper by means of a solenoid, but the paper is automatically lifted up so as to touch the pen. This avoids the danger of magnetic disturbances caused by having a solenoid plunger mounted on the pen arm, and also reduces the moment of inertia of the moving system to a minimum. Owing to the additional energy required to raise the paper as well as to advance it, this double function is performed by an electro‐magnet mounted at a suitable distance below the instrument. The clock that periodically closes the electric circuit may then be located at any distance from the instrument. Other changes and new features are described in the following paragraphs. With the exception of the enclosing cases and the clockwork, practically the entire work of construction was carried out by Mr. F. H. J. Newton, the college mechanician, to whose ingenuity and skill acknowledgement is here made.