TERRESTRIAL MAUNKTISM. 



TEKKIXI'KI M, MAGNETISM. 



100 



Itrtfrminntlum of lie ll<*i:mtal Component of Iht ifagnetit Pont. 

 Thin element ii obtained by a combination of two experiments made 

 with an instrument called the t'nililar Magnetometer. In the first 

 experiment, tl..- time i>f vibration of a suspended magnet is observed. 

 In the wcond experiment we observe the amount of deflection which the 

 BUM magnet produces in a second magnet suspended by a single thread. 



Let T denote the time of vibration of a suspended magnet, deter- 

 mined much in the same way as the oscillation of a pendulum might be 

 observed, that i, by observing the time of the passing and repassing of 

 a fixed point on the magnet past a vertical line placed in the focus of a 

 fixed telescope. Let M be the magnetic moment of the liar, K iU 

 moment of inertia, then by the principles of Dynamics we have 



Next let this same bar , be placed on a support in such a position 

 that its axis lies in a straight line, passing through the centre of 



another magnet s' s', suspended by a single thread, and its direction 

 perpendicular to the axis of the latter magnet. Then the magnet K s 

 will deflect the magnet K' s' from its normal position into a position 

 >" s". Let u denote this angle of deflection N'CN", r the distance 

 between the centres of the two magnets. Then the condition of equi- 

 librium will be given by the equation 



on i 



(2) 



combining this with equation (1), we hare 



w /~2x 

 x= ir' V Trin" 



HI ; . 



V 4 r K mn u . 



It is to be observed that the unite of time, space, and mass referred 

 to in these investigations, are by universal consent taken to be, a 

 second, a foot, and a grain respectively. 



But though we are thus enabled to determine the absolute value of 

 this horizontal component X, and, therefore, also the changes it under- 

 goes from time to time, yet it is not adapted to give those very minute 

 changes which take place from hour to hour, and day to day. Such 

 changes are called horary and diurnal variations ; and the instruments 

 by which they are observed are called differential instruments. The 

 variation in the horizontal force is denoted by the symbol A x ; and is 

 observed by means of an instrument called a Bid lor Magnetometer. 



K s is a magnet suspended by two threads hanging from a screw 



which can be turned horizontally, so that the axis of the magnet can 

 be constrained to assume a position approximately perpendicular to 

 the magnetic meridian. Beneath the magnet, a graduated transparent 

 scale, c, is suspended, and the readings of this scale are viewed by 

 means of a telescope, T, whose axis is in the magnetic meridian. Now 

 the horizontal force, x, evidently acts perpendicularly to the magnet, 

 hence every change that it undergoes will cause the magnet to be 

 deflected from its normal position. These deflections are read off by 

 means of a vertical wire placed in the focus of the telescope ; hence, 

 having determined the change in the horizontal force which corres- 

 ponds to one division of the scale, the reading of the scale at any time 

 will show us the increase or decrease of the horizontal intensity the 

 zero reading of the scale corresponding to the normal value of tbu 

 horizontal force. 



Tht Vertical Fvrct Magnetometer is another differential instrument 



for observing A r, a small variation in the vertical component v. It 



consists of a light magnet, x a, with small holes at its extremities, 

 across which fine threads are drawn in a horizontal position. Through 

 the centre of the magnet is a knife-edge, c, perpendicular to the axis 

 of the bar. This knife-edge is supported by two smooth agate planes, 

 so as to free the motion of the magnet, as far as possible , from the 

 effects of friction. The axis of the bar is placed in a position perpen- 

 dicular to the magnetic meridian, thus rendering; it independent of the 

 inclination or dip. A small shifting weight is attached to the magnet, 

 and is so adjusted as to bring the axis into a horizontal line for its 

 normal position. Now it U evident that any change in the vertical 

 force will deflect the bar from its horizontal position, and this deviation 

 is viewed by means of two ordinary micrometers directed to the holes 

 at x and 8. The change in the vertical force which corresponds to 

 any number of revolutions of the micrometer screw may be found 

 by absolute determination; hence, also, the changes due to one or more 

 divisions are known the zero reading of the micrometer corresponding 

 to the normal position of the axis of the magnet, for which position the 

 absolute value of the vertical component T has been carefully deter- 

 mined from the formula v = x tan 9. 



The luclinniinn or Dip is determined by means of a light magnet 

 whose extremities taper to a point, and through tUe centre of which is 



a smooth cylindrical axle perpendicular to the plane in which the axis 

 of the magnet moves. This circle rests on two smooth agate planes, 

 and thus the magnet moves freely in a vertical circle whose rim is 

 graduated through each of the four quadrants from to 90. When 

 used, the instrument is adjusted so that the plane of the graduated 

 circle, together with the axis of the magnet, is in the plane of the 

 magnetic meridian, previously determined. The needle now assumes 

 the direction of the magnetic force at the place of observation, and the 

 reading of the graduated circle opposite to either extremity of tin: 

 magnet indicates the inclination of the axis of the needle to the 

 horizon. In practice, the means of both readings are taken, and several 

 pairs of observations made. These and many other precautions are 

 adopted in order to eliminate, as far as possible, the mechanical defects 

 ill the instrument, and errors of observation. 



The Declination or Variation is determined by the following very 

 simple process : Let c T denote the magnetic meridian ; c r, the astro- 



nomical meridian. Then the axis of a magnetic bar, ACS, suspended 

 by a single thread attached to a point atimc its centre of gravity, will 

 assume a horizontal position in the line c T ; and a graduated glass 

 scale being attached to the magnet, its cnro-reailin^. \\ !ii<-h is token to 

 indicate the normal position of the magnet, is made to coincide with a 

 vertical wire in the focus of the fixed telescope T. The angle r c T is 

 the declination or inclination of the magnetic to the astronomical 

 meridian, and is measured by a theodolite which is placed so that its 

 axis is made to coincide with the line CT ; and being then turned in 

 azimuth until a distant object is seen which is tcnovn to be in the 

 meridian of the place, the acunutlial angle thus measured is the normal 

 declination. 



This instrument, which is sometimes called the Unifilar Magneto- 

 meter, likewise serves to make differential observations of the declina- 

 tion ; for there is no difficulty in determining the angular value of 

 each division of the scale attached to the magnet: hence, if the vertical 

 wire hi the telescope coincides with the zero-reading of the scale at any 

 specified time, when the noodle is supposed to be in its normal position, 

 any other reading will denote the angular deviation of the magnet from 



