Sept. 21, 1882] 



NA TURE 



507 



In the bridge connecting the two sides of the frame a 

 sapphire cap is fixed, and this rests on an iridium-tipped 

 point standing up from the bottom of the containing 

 box. The sides of the frame are made long enough to 

 form when brought together at one end an index about 

 nine centimetres long of the shape shown in Fig. 2. The 

 point of the index ranges round a scale of tangents 

 placed round the curved edge of the bottom of the box. 

 To prevent error from parallax the bottom of the box, with 

 the exception of the narrow strip occupied by the scale, 

 is covered with a mirror of silvered glass. The observer 

 when taking a reading places his eye in such a position 

 that the point of the index just covers its reflected image, 

 and reads off the deflection indicated by the position of 

 the point of the index on the scale of tangents. The scale 

 is engraved on paper, and firmly fixed to the bottom of 

 the box by photograper's glue ; and thus any change of 

 length due to varying amount of moisture in the 

 atmosphere is avoided. 



The magnetometer box rests on three feet and a flat 

 spring. Two of these feet, which are in a plane perpen- 

 dicular to the plane of the box and passing through the 

 supporting point and the zero of the scale, slide in the V 

 groove cut along the middle of the platform : the third 

 foot rests on the plane surface on one side of this 

 groove, the spring on the other side. By this arrange- 

 ment the magnetometer is rendered perfectly steady and 

 can be moved with perfect freedom along, but only along 

 the platform. A small circular level carried by the box 

 shows when the plane of the magnetometer is horizontal. 

 This adjustment is made by means of the two screws 

 which support the platform at the end remote from the 

 coil. 



To lift the system of magnets and index off the bearing 

 cap when the instrument is not being used, or when it is 

 being carried from one place to another, a small collar- 

 piece free to move round the supporting point is raised 

 up by a horizontal screw turned by a head outside the 

 magnetometer box. When raised this collar-piece forms 

 a supporting platform for the needles and se:urely 

 prevents them from moving about and sustaining damage. 



To increase the directive force on the needles when 

 required, the semi-circular magnet shown in the drawing 

 is used with the instrument. This magnet is made of the 

 best steel, and is tempered glass-hard. It is magnetised by 

 sending a current through a semi-circular coil containing it. 

 When in position on the instrument it is supported on 

 two flat pieces of brass projecting from the radial sides 

 of the magnetometer box. The magnet terminates at 

 one end in a cross piece of brass having on its under 

 side at one end a small projecting brass knob. This 

 knob fits into a hollow in one of the projecting arms of 

 brass, while the other end of the cross piece rests simply 

 on the plane surface of the arm. The other end of the 

 magnet is brought to a rounded point which rests 

 in a V notch cut round a cylindrical shoulder on a 

 screw spindle (seen on the right-hand side of Fig. 2), 

 which works through a nut fixed to the other projecting 

 arm of the magnetometer box. The magnet thus rests 

 with its magnetic axis as nearly as may be in the hori- 

 zontal plane through the axis of the needle, and nearly 

 at right angles to the line joining the centre of the 

 needle's axis with the zero of the scale. Its axis may 

 be placed accurately at right angles to this line by turning 

 the screiv until the needle points accurately to zero. 

 The magnet thus mounted remains in the same position 

 relatively to the magnetometer. 



The coil is so adjusted that its centre is on a level with 

 the magnetic axis of the needle when the magnetometer 

 is in position. The centre of the magnetic axis, the zero 

 of the scale, and the horizontal V groove in the platform 

 are in the vertical plane through the centre of the coil. 

 Hence if the magnetometer guided by its feet in the v 

 groove be moved along the platform it will carry its 



magnet with it without disturbing its zero adjustment of 

 the needle, and the magnets will in every position of the 

 magnetometer be in the same field of force. 



On the boxwood slip in which the V groove is cut is 

 marked a series of positions of the front or circular edge 

 of the magnetometer, for which the corresponding num- 

 bers of divisions of deflection for one volt difference of 

 potentials, when the intensity of the magnetic field at the 

 needle is one C.G.S. unit, are the terms of the geometric 

 series . . . 8, 4, 2, I, £ . . . These numbers are stamped 

 on the boxwood slip opposite the marks indicating the 

 corresponding positions. The number of divisions of 

 deflection for the nearest position of the magnetometer, 

 that at which the centre of the magnetic axis of the needle 

 is as nearly as may be at the centre of the coil, is not 

 generally a term of this series, but it is determined 

 in every case, and like the others is stamped on the 

 platform. 



The instrument is used for the measurement of high 

 potentials with the semicircular magnet in position ; but 

 for low potentials the magnet is dispensed with, and the 

 needle left under the earth's directive force alone. The 

 field intensity given by the magnet of each instrument is 

 determined before the instrument is sent out, and is 

 painted on the magnet. The intensity of the field without 

 the magnet, at the place at which the instrument is used 

 has if necessary to be determined. In practice it will 

 generally be found convenient to use some position of the 

 magnetometer which gives a convenient number of 

 divisions of deflection per volt for the field employed. 

 This position is determined by the user of the instrument, 

 who marks it on the platform by drawing two vertical 

 lines on the sides so as to prolong two white lines which 

 are marked on the sides of the magnetometer. 



The instrument as thus constructed admits of a very 

 wide range of sensibility. By diminishing the distance 

 of the magnetometer from the coil from the greatest to 

 the least, the sensibility of the instrument can be increased 

 fifty fold : and by removing the field magnet from the 

 instrument and leaving the needle under the influence of 

 the earth's force alone, a sensibilityfiftythr.es still greater 

 can be given to it. For the practical purposes for which 

 these instruments are designed the suspension of the 

 needle by cap and point is the most convenient ; but 

 with this suspension there is alwavs, with low directive 

 forces, a slight error due to friction : and it is therefore 

 not advisable to push the sensibility of the instrument 

 further by diminishing the directive force of the earth's 

 magnetism. An instrument of this kind, however, made 

 for special purposes with a silk fibre suspension could be 

 rendered more and more sensitive up to the limit of 

 instability by so placing a magnet or magnets as, while 

 not interfering with the uniformity of the field at the 

 needles, to diminish more and more the earth's directive 

 force. This method of increasing the sensibility of a 

 galvanometer although quite commonly used by scientific 

 electricians is not, I have reason to believe, at all well- 

 known generally, and recourse is had, altogether unneces- 

 sarily in many cases, to troublesome astatic combinations 

 in order to obtain sensibility. 



An important feature of this instrument in connection 

 with its use f-r the measurement of high potentials is 

 the arrangement of terminals which has been adopted. 

 In certain circumstances when the ends of the coil 

 of a potential galvanometer are attached to terminals 

 fitted with binding screws, it is convenient to con- 

 nect the instrument with the circuit by wires attached 

 to these screws ; but in the case of a dynamo circuit 

 giving between the terminals of the coil a potential 

 difference of eighty or a hundred volts and upwards, 

 this plan of connections has been found highly dan- 

 gerous. If the wires are twisted together and are 

 ordinary gutta percha covered wires there is always a 

 liability to accidents which may cause conduction from 



