113 



Why do different Magnetic Needles not always point 

 in the same direction, though observed at the same 

 place and time ? 



A close observer of the compasses used in surveying instruments must have 

 noticed that the exterior shape of the magnetic needles, forming part thereof, frequently 

 differs as much as do the styles of the instruments themselves. In some cases the 

 needle consists of a bar of thin steel, oblong in shape, which rests flatwise on its sup- 

 porting pin, so that its greatest superficial area lies in the horizontal plane as shown in 

 Fig. 1. In others, as represented in Figs. 2, 3 and 4, the shapes are modifications of 

 the above in a greater or less degree, inasmuch as their longer transverse dimensions 

 whether at the ends or center also lie in the horizontal direction. Some magnetic 

 needles carry a graduated circle or verniers, as the case may be, as seen in Figs. 5 and 

 6, whose zero points are supposed to lie in the geometrical axis of the needle ; some are 

 placed edgewise, so that the greatest superficial area lies in the vertical plane. There 

 are also dipping needles to measure the vertical intensity of the earth's magnetism ; 

 short and stubby needles with aluminum extensions for galvanometers ; complex nee- 

 dles, made in sections, used in marine compasses, and cylindrical or tubular forms used 

 in scientific research. (Needles of the last mentioned type are not read by observing 

 the ends, but by means of mirrors attached at the middle similar to those on a magnet- 

 ometer. ) Figs. 9, 10 and 11 show the principal types of the last mentioned needles.* 



Since these latter forms are never met with in surveying instruments, we shall not 

 consider them here, except to show the different styles of needles in vogue suffice it 

 to say that some of the shapes are simply selected by their propounders for the larger 

 superficial area which they have, compared with others, according to the purposes for 

 which they are intended ; for it is well known that the larger the superficial area, com-, 

 bined with a minimum weight, the more delicate will the magnetic needle be ; thus, a 

 needle made of a very thin steel tube will be capable of receiving and retaining a greater 

 charge of magnetic force, and, also being very light preventing wear of the cap and 

 pin it is easily influenced to assume the direction of the magnetic meridian in 

 azimuth. 



However, we wish to remark here that it is not so important that the needle of a 

 surveying instrument should have a great magnetic intensity up or nearly to saturation 



which it may receive according to its superficial area and degree of hardness as it 

 is that it should have as constant an amount as possible, be it great or small. Of 

 course such a needle should not have a surcharge, which it is apt to receive if im- 

 properly hardened, because its intensity is liable to be diminished and the needle 

 thrown out of balance thereby, from time to time requiring a readjustment of its 

 counterpoise for the same latitude in which it is used. 



It is all-essential that a well constituted needle should have a proper symmetrical 

 form as regards the longitudinal axis, and that it should be supported on a pivot as free 

 from friction as it is possible for human ingenuity and skill to contrive. Besides the 

 necessity of proper form and suspension, the quality and degree of hardness of the 

 steel, length of the needle, and lastly the strength of its magnetic force, whether 

 imparted by an auxiliary magnet or the more powerful electric battery and coil, have a 

 most important influence on the behavior of a needle. 



lleturning to Fig. 1 we see exemplified the flat bar, and in Fig. 7 the edge bar 

 needle as used in surveyors' compasses, but as all the intermediate styles of needles are 

 simply modifications of the flat oblong form in order to be light in weight, partaking 

 of the same principle that is involved, we shall consider these two forms almost exclu- 

 sively. It remains therefore, for us to show in how far the exterior shape of a needle 



whether of a faulty design or imperfect construction may affect its reading, inas- 

 much as its geometrical axis, that is, the line passing through its ends and the center 



* The largest and most delicate magnetic needle with which the writer is acquainted is that of the large 

 magnetometer in the University of Marburg, made of a solid bar of steel two feet long and weighing about 

 twenty-five pounds. It was suspended from the ceiling by a strand of silk fibers, and was provided with 

 mirrors so that any oscillation could easily be read by the use of a scale and theodolite. This instrument was 

 made after the style of Gauss' instrument, and with it many observations were made to determine the diurnal 

 and annual changes in the magnetic meridian at that place during the younger days of the writer. 



