304 Professors Ayrton and Perry on Direct-Reading 



to see whether the argument for the comparative coolness of 

 spots based on the total radiation is irrefragable; and we have 

 concluded that it is not, even after taking into account the 

 considerations very properly adduced by Professor Wiede- 



mann. 



XXXV. Direct-Reading Electro-Measuring Instruments, and a 

 Non-Sparking Key. By Professors W. E. Ayeton, F.R.S., 

 and John Pekky, ^/.i?.* 



[Plate XII.] 



THE construction and use of our various measuring- 

 instruments are now so sufficiently well known that it is 

 unnecessary for us to give any general description of them ; 

 indeed the names " Ammeter/' " Voltmeter/' by w T hich we 

 ventured to christen two of our children, have found favour 

 in the eyes of Electricians, and are now current in electrical 

 literature as the typical names, or surnames, of instruments 

 used to measure strong currents and large electromotive forces 

 respectively. But just as improvements in electrical measure- 

 ments have led, step by step, to the names for the electrical 

 units receiving more and more exact definitions, so we now 

 propose to take advantage of certain important improvements 

 that we have recently effected in our measuring-instruments 

 to define the names u Ammeter/' "Voltmeter," and "Ohm- 

 meter " more exactly. 



Proportional Laic. — In the earliest form of instruments 

 commercially employed for measuring the large currents used 

 for electric lighting, viz. the instruments of M. Deprez, refer- 

 ence had to be made to a table of values to ascertain the 

 meaning of any particular deflection, since there was no im- 

 mediate connexion between 1 degree deflection and 1 ampere; 

 and, further, it was necessary to refer to the table twice over 

 when measuring two different currents, as the deflections 

 were not proportional to the current. By giving a proper 

 shape, however, to the coil, needle, and pole-pieces of the 

 controlling permanent magnet, we succeeded (in 1880) in 

 producing instruments in which the current or potential dif- 

 ference was exactly proportional to the deflection throughout 

 the whole range of the scale; and, further, by the employment 

 of our commutator principle an instrument was obtained 

 having two degrees of sensibility, one exactly ten times the 

 other; so that a commutator ammeter, or voltmeter, although 

 employed to measure strong electric currents and large po- 

 tential differences respectively, could be calibrated by the 

 * Communicated by the Physical Society. Read January 26, 1884. 



