310 Professors Ayrton and Perry on Direct-Reading 



Methods of Construction for obtaining the Proportional Law. 

 - — Next, with reference to the principle that must be followed 

 to obtain the proportional law in galvanometers. In an ordi- 

 nary galvanometer controlled by the earth-magnetism, the 

 strength of the controlling field is a uniform one; whereas the 

 strength of the deflecting field produced by the current flow- 

 ing round the coil is strongest in the plane of the coil, and 

 grows weaker as we recede from that plane. Hence, when 

 the needle is deflected, it moves into a weaker part of the 

 deflecting field. The result of this is that the deflections for 

 different currents increase much less rapidly than the current. 

 If both fields be uniform for all positions of the needle, and be 

 at right angles to one another when the needle is in its zero 

 position, then, as is well known, the tangent of the deflection 

 increases proportionately to the current. In order, then, that 

 the deflection may increase proportionately with the current, 

 one of three conditions must be fulfilled: — (1) the needle when 

 deflected must move into a stronger part of the deflecting- 

 field, or (2) must move into a weaker part of the controlling- 

 field, or (3) both these results must be true. The first con- 

 dition (uniformity of controlling-field combined with increase 

 of strength of deflecting-field) can be fulfilled by using two 

 solenoids at a small distance apart from one another, but with 

 a common axis, and suspending the needle between them so 

 that, on being deflected, it enters the solenoids and therefore 

 moves into a stronger part of the solenoid field ; or it may 

 be fulfilled by a plan carried out practically by two of our 

 assistants, Messrs. Walmsley and Mather, and which consists in 

 suspending a magnet over two coils, C C, wound as shown by 

 the dotted lines in fig. 4, the distance between the coils being 

 rather less than the length of the needle. 



This latter mode has been employed in our laboratory for 

 making students' rough galvanometers, a specimen of which 

 you see in its embryo state on the table, and with which the 

 deflections are accurately proportional to the current at any 

 rate up to 45° or 50°. If, however, a dead-beat instrument 

 is required, then, in order to ensure roughly uniformity of 

 controlling-field, the pole-pieces, P P, of the permanent mag- 

 net, M M, must be hollowed out, as shown in fig. 5, and must 

 embrace the tube, T, in which the needle moves. We have 

 made a number of ammeters and voltmeters of this type, and 

 a sample one dissected is on the table. The augmentation of 

 strength of the deflecting-field is still further produced by 

 the employment of the soft-iron cores F F referred to above, 

 and which screw into the brass bobbin A A. 



But the condition 3 (the needle moving into a stronger part 



