Electricity for public schools and colleges . the needle, and passing through the centre of the graduated card,turns a telescope. This telescope is provided with an object-glass so made that both distant and near objects can be viewedwithout altering the focus of the instrument. It is capable ofbeing inclined to the horizontal plane as well as of turning abouta vertical axis. There is a fixed horizontal graduated circle and an indexmovable with the telescope, to tell us through what angle we turnthe telescope about its vertical axis. 32 ELECTRICITY en. III. The general method, without detail,
Electricity for public schools and colleges . the needle, and passing through the centre of the graduated card,turns a telescope. This telescope is provided with an object-glass so made that both distant and near objects can be viewedwithout altering the focus of the instrument. It is capable ofbeing inclined to the horizontal plane as well as of turning abouta vertical axis. There is a fixed horizontal graduated circle and an indexmovable with the telescope, to tell us through what angle we turnthe telescope about its vertical axis. 32 ELECTRICITY en. III. The general method, without detail, is as follows. With the telescope observe some distant object whose geo-graphical bearing is known, some star not far from thehorizon. Read off the position of the index on the graduatedcircle ; and, from knowledge of the bearing of the object, notedown at what division on the graduated circle the index wouldstand if the telescope were directed to the geographical set the telescope parallel to the magnetic axis of the needle,. Fig. i. which, if the instrument be perfectly made, will be done by turn-ing the telescope round till the end of the needle is the position of the index. The difference between thisreading and the last will be the angle between the magnetic axisof the needle and the geographical north and south line, or it willbe the declination. Corrections.—(i.) If the vertical axis about which the telescope turnsdoes not pass through the point of suspension of the needle, then the CH. III. MAGNETIC MEASUREMENTS 33 telescope, when the whole instrument is viewed from above, would beseen to lie off to one side of the needle. The consequence of thiswould be that when the telescope was directed so as to point at oneend of the needle, it would be inclined to the needle and not parallelto it. In fig. ii. (which represents this state of things as viewed directlyfrom above, or is a horizontal projection of the needle and optical axisof the telescop
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Keywords: ., bookcentury1800, bookdecade1890, bookpubl, booksubjectelectricity
