no Instruments. 307 



a important, to avoid complexity of construction, that the 

 adjustment for sensibility referred to above should, if possible, 

 be so contrived as to serve also for an adjustment for the pro- 

 portional law. And this we are happy to say we have at last 

 succeeded in accomplishing, by fitting into each end of the 

 bobbin round which the wire is wound specially constructed 

 charcoal-iron cores fitted with a fine screw. If these cores be 

 too tar out or out altogether, the proportional law will pro- 

 bably not be true, the current increasing more rapidly than 

 the deflection ; on the other hand, if they be screwed fully 

 home the proportional law will also not be true, the deflection 

 now increasing more rapidly than the current ; but we find 

 that between two limiting positions of these iron cores the 

 proportional law is true: and bv moving the cores in or out 

 within these limits, the sensibility of the instrument can be 

 much altered without destroying the proportional law. 



To construct then. say. .* - .rnpere instrument, wire is 

 selected of such thickness, first, that 50 amperes will not heat 

 it too much ; secondly, that a sufficient number of convolu- 

 tions can be wound on the bobbin for 50 amperes to det: -:■;: 

 the needle to the limit of the scale when it is controlled by a 

 permanent magnet of normal power. A scale divided for ' 

 amperes (that is, having the division marked 50 amperes at 

 the end of its range) is then put on, and the soft-iron c; - 

 screwed in or out a little by trial until the pointer is found to 

 point to exactly the same number on the dial as the number 

 of amperes of current flowing through the coil. If at any 

 time it be found that the permanent magnet has lost some of 

 its magnetism, so that less than, say, -k> amperes will deflect 

 the pointer to 40 amperes on the dial, the iron cores are un- 

 screwed out a very small distance until the change in the 

 magnetism of the controlling magnet is exactly compensated 

 for. 



The other adjustment to be employed by the makers for 

 equalizing the deflections right and left for the same current 

 is effected by fixing the coil so that it can turn about its 

 centre. 



The ,; Ten Law" of our commutator-instruments, which 

 means that an ammeter is exactly ten times as sensitive when 

 the commutator is turned to series as when to parallel, while 

 a voltmeter is ten times as sensitive when the commutator is 

 turned to parallel as when turned to series, is effected by 

 the ten wires forming the bobbin having, inclusive of the 

 resistance of the contacts at their ends, exactly the same res Fr- 

 ance; which condition being fulfilled, it is quite unnecessary 

 for obtaining the ten-law that the ten wires composing the 



