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Encyclopedia Britannica - Main :: HEG-HIG |
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HELIOSTAT (from Gr. rtXtor, the sun, vrares, fixed, set up) , an instrument which will reflect the rays of the sun in a fixed direction notwithstanding the motion of the sun. The optical ' apparatus generally consists of a mirror mounted on an axis
axis
celestial
' Dollond
Many forms of heliostats have been devised, the earliest having I been described by Wilhelm Jacob s'Gravesande in the 3rd edition of his Physices elementa (1742). One of the simplest consists of a I plane mirror rigidly connected with a revolving axis so that the angle between the normal to the mirror and the axis of the instrument equals half the sun's polar distance, the mirror being adjusted so that the normal has the same right ascension as the sun. It is easily seen that if the mirror be rotated at the same angular velocity as the sun the right ascensions will re-main equal throughout the day, and therefore this device reflects the rays in the direction of the earth's axis; a second fixed mirror reflects them in any other fixed direction. Foucault's heliostat reflects the rays horizontally Fin. t. in any required direction. The principleof the apparatus may be explained by reference to fig. 1. The axis of -rotation AB bears a rigidly attached rod DBC inclined to it at an angle equal to the sun's polar distance. By adjusting the right ascension of the plane ABC and rotating the axis with the angular velocity of the sun, it follows that BC will be the direction of the solar rays throughout the day. X is the mirror rotating about the point E, and placed so that (if EB is the horizontal
which the rays are to be reflected) (t) the normal CE to the mirror is jointed to BC at C and is equal in length to BE, (2) the rod DBC passes through a slot in a rod ED fixed to, and in the plane of, the mirror. Since CE equals BE these directions are equally inclined to, and coplanar with, the normal to the mirror. Hence light incident along the direction BC will be reflected along CE. Silbermann's heliostat reflectsthe rays in any direction. The principle may be explained by means of fig. 2. AB a. is the axis of rotation, BC an adjustable rod as in Foucault's construction, and BD is another rod which can be set to the direction in which the rays are to be reflected. The rods BC and DB carry two small rods EF, GF jointed at F; at this joint there is a pin which slides in a slot on the rod BH, which is normal to the mirror X. Theto the framework pate, the members of which are parallel to the incident and reflected rays SO, OR, and the diagonal pf is perpendicular to the mirror. The framework is attached to two independent circular arcs Cs and rr' having their centres at 0 and provided with clamps D and A on the axis F of the instrument. The arc Cs is graduated, and is set so that the angle COD equals the complement of the sun's declination. This can be effected (after setting the axis) by rotating Cs until a needle indicates true time on the hour dial B. The arc rr is set so as to reflect the rays in the required direction. The axis F of the instrument is set at an angle equal to the latitude
latitude
Many other forms of heliostats have been designed, the chief
horizontal
End of Article: HELIOSTAT (from Gr. rtXtor, the sun, vrares, fixed, set up) If you wish, you can link directly to this article.
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