306 Professors Ayrton and Perry on Direct-Reading 



On the table there are various specimens of our new Direct- 

 Reading Ammeters and Voltmeters. In each case the dial is 

 a complete disk (see Plate XII. fig. 1), and not a sector as 

 in the old instrument. The light aluminium pointer is also 

 much longer, and the length of the graduated arc over which 

 its end moves is much greater, so that readings with the new 

 instruments can be made much more accurately than in the 

 old. Here is a direct-reading ammeter suitable for measuring 

 up to 10 amperes, correct to a tenth of an ampere ; here an- 

 other for measuring up to 150 amperes, each division of the 

 scale corresponding with one ampere. I now send a current 

 with these accumulators through this thick German-silver 

 wire, and you see at once from the deflection on this ammeter, 

 without any reference to a table of values, and without the 

 employment of any troublesome constant, that the current 

 passing, and which is now making the wire white-hot, is 207 

 amperes. Again, each division on the scale of this voltmeter 

 corresponds with 1 volt, this particular instrument being suit- 

 able for measurements up to 100 volts. 



To construct a direct-reading instrument it is necessary to 

 employ some form of adjustment for sensibility, since to have 

 a special scale engraved for each instrument after it was wound 

 would be boil) troublesome and expensive ; and, further, if 

 there were no power of adjustment, any change in the strength 

 of the permanent controlling magnet would prevent the in- 

 strument from any longer giving one division-deflection per 

 ampere or per volt — in other words, would necessitate the 

 employment of a constant. 



Difficulties of Manufacture of Older Forms. — As regards the 

 two defects from tlie manufacturer's point of view they were 

 these: — Even although the shape of the coil, needle, and pole- 

 pieces of the magnets were made accurately to templates, it 

 was found difficult, when very thick wire had to be coiled on 

 the bobbin of an instrument formeasuring strong currents, to 

 wind it with certainty so that both the deflection was propor- 

 tional to the current, and that equal deflections on the two 

 sides of the zero had exactly the same value. As long as the 

 wire was thin, symmetry of winding was comparatively easy 

 to obtain; but when thick wires, having but a lew convolu- 

 tions, were wound round a small bobbin, the tliiekness of the 

 wire, combined with Hie galvanometric importance of each of 

 ihe few convolutions, especially of those closest to the needle, 



made il necessary to wind and rewind some of ihe instru- 

 ments several limes before the proportional law, as well as 

 equal deflections right and left, could be obtained. 



Adjustment of Direct- Reading Instruments. — It therefore 



