486 Mr. T. Gray on Measurements of the Intensity of the 



sary by making the deflected magnet so short that its magnetic 

 length may be neglected ; and, in the second place, the effect 

 of the length of the deflector is found by adopting two posi- 

 tions for which the length enters in the resulting equations (4) 

 and (5) above, with opposite sign, rendering' the observation 

 highly sensitive to that effect. By adopting this method a 

 good estimate of the effective length of different deflectors is 

 obtained ; and this is useful information outside of the parti- 

 cular object of the experiment. 



The latest form and arrangement of the apparatus for the 

 deflection-experiment is shown in PI. XI. fig. 1, where 

 T is the table on which the apparatus is placed, M the 

 magnetometer, A and B the deflector-stands, and the scale 

 on which the deflections are read. The magnetometer M 

 consists of a light mirror about *8 centim. in diameter, on the 

 back of which two magnets, 1 centim. long and '08 centim. in 

 diameter, are fixed. A better form of needle would, as has 

 been pointed out by Sir William Thomson, be two thin disks 

 of hard steel mounted with their planes parallel and at a dis- 

 tance apart somewhat less than the diameter of the disk. 



The mirror with attached magnets is suspended by a single 

 silk fibre (half a cocoon-fibre) in a recess cut in a block of 

 wood, W. Tw t o holes at right angles to each other, and pass- 

 ing through the position of the mirror and magnets, allow the 

 magnetic system to be accurately adjusted when setting up 

 the apparatus. The holes, with the exception of that in 

 front of the mirror, are plugged while the instrument is in 

 use, and the mirror and fibre are protected from currents 

 of air by means of a plate of plane glass. The sole plate, 

 P, is furnished with three brass feet, which rest on a " hole, 

 slot, and plane " arrangement cut in the top of the glass 

 plate, p, fixed to the table. The deflector-stands, A and 

 B, consist of a base plate of mahogany furnished, as in the 

 case of the magnetometer, with three brass feet which rest on 

 " hole, slot, and plane " arrangements cut in the glass plate p. 

 A centre pivot, c, is fixed in the sole plate and passes through 

 a closely-fitting hole in the glass plate, g y which rests on three 

 thin blocks of hard wood and is free to turn in azimuth. A 

 strip of wood having a V-groove cut along its upper side and 

 furnished with an adjusting-screw, s, is cemented to the top of 

 the glass plate g in such a way that the bottom of the V-groove 

 is vertically above the centre of the pivot c. The screw, s, gives 

 an adjustment for the centre of the magnet, which is necessary 

 if the magnets differ slightly in length. The centre of the 

 magnet is seldom exactly at the middle of the length of the 

 bar, but, with the arrangement here adopted, the adjustment 



