88 The Rev, Dr. Callan's Experiments on the 



always kept in vertical position ; and of a compass-box, which, 

 by means of a slide 3 feet long and at right angles to the circle 

 at its centre, may be moved in a direction perpendicular to the 

 circle to the distance of 3 feet from it, so that the centre of the 

 needle, which is a bar-needle 5^^ inches long, will always be in 

 the axis of the circle and of the coil, and that the Une joining 

 the north and south points of the compass-box will be always 

 parallel to the horizontal diameter of the mahogany circle and 

 coil. From this description of the instrument, it is evident that 

 {no matter where the compass-box is placed on the slide) the 

 needle is parallel to the mahogany circle and coil, or perpendi- 

 cular to their axis, whenever it points to 0°. Hence, if a voltaic 

 current, sent through the coil, deflect the needle, and if the 

 cu'cle and coil be turned round so as to follow the needle until 

 it points to 0°, the needle, no matter where it may be placed on 

 the slide, will then be parallel to the coil and perpendicular to 

 its axis. The magnetic power of the coil is exerted in the direc- 

 tion of its axis. The effective part of the earth's magnetism in 

 impelling the needle to the magnetic meridian is also exerted in 

 the direction of a perpendicular to the needle or of the axis of 

 the coil, but opposite to that in which the magnetic force of the 

 coil acts. Since the needle is kept at rest by these two forces 

 acting in opposite directions, they must be equal. But the 

 effective part of the earth's magnetism in impelling the needle 

 to the magnetic meridian, varies as the sine of the angle which 

 it makes with that meridian. Therefore the magnetic power of 

 the current flowing through the coil also varies as the sine of the 

 angle which the needle, wlien it points to 0°, or the mahogany 

 circle D, makes with the magnetic meridian. If the connexion 

 with the battel^ be broken, the needle will immediately return 

 to the magnetic mei'idian. The graduated circle of the compass- 

 box will give the number of degrees the needle was deflected 

 from the magnetic meridian. For measuring the angle of 

 deviation, I have used a large graduated circle about 13 inches 

 in diameter, which is attached to the upper part of the mahogany 

 circle, and at right angles to it and to the axis about which it is 

 moveable. By means of seven brass bolts, each three-eighths of 

 an inch in diameter, I can send the current from the battery 

 through one coil, or seven, or through any intermediate number 

 of coils. I scarcely ever use more than one coil, and that is the 

 outside one, the diameter of which is about 2 feet 2 niches. 

 AVhen the current is sent through seven coils, the deflection is 

 so great that only very feeble currents can be measured on the 

 sine galvanometer. A\'lien the needle is in the centre of the 

 coil, this galvanometei-, used as a sine instrument, lai-ge as is its 

 diameter, is not capable of measuring the power of a current 

 produced by a single circle of the cast-iron battery, in which the 



