xxxii Introduction. 



§ 5. Balance oe Veetical Foece Magnetometee. 



36. The Balance Magnetometer is by Bobinson of London. It is composed 

 of a needle 12 inches long, and about 1 inch broad, with knife-edge axle, resting on 

 agate planes ; at the extremities of the needle are brass rings, each carrying a cross 

 of spider's threads. The needle is placed horizontally, at right angles to the mag- 

 netic meridian It is accurately adjusted by means of two fine brass screws; one 

 working horizontally towards one extremity balances the needle, the other near the 

 other extremity, working vertically, regulates its sensibility. 



The apparatus is covered by a rectangular box, having glazed openings on two 

 sides, opposite the spider's crosses ; those on one side allowing light to be thrown 

 on the crosses from two small mirrors, those on the other for viewing them, and 

 determining their position, which is done accurately by means of microscopes car- 

 rying micrometers. There is a thermometer within the box for giving the tempe- 

 rature of the magnet. 



37. If m be the moment of free magnetism of the needle, Y the vertical com- 

 ponent of the earth's magnetic force, W the weight of the needle, g the distance 

 from the centre of motion to the centre of gravity, e the angle contained by the line 

 joining these two centres, and the magnetic axis of the needle on the horizontal, the 

 magnetic axis being at right angles to the plane of the magnetic meridian ; then the 

 equation of equilibrium is 



mY = W^cose 



Whence differentiating the equation and dividing by it 



A Y , Am 



■^—- = tan e A € 



Y m 



A €, or the varying angle which the magnetic axis makes with the horizontal, is 



obtained from observations with the micrometer ; for see section on temperature 



correction. 



It is conceived that f cannot be determined with accuracy by the practical 

 method of inversion, and Dr Lloyd has shewn that it may be obtained from the 

 following formula : 



tan 6 = cot 6-^ 



T' and T being the times of one vibration, the former in the horizontal plane, 

 the latter in the vertical plane, and & the magnetic dip. 



38. Adjustments, &c. 



June 16, 17. 1841. Experiments were made to determine the effect of tem- 

 perature on the balance magnet. See subsequent section. 



39. Values, in arc, of the divisions of the micrometer heads. A brass needle 



