478 Profs. Ayr ton and Perry on Incandescent 



while an incandescent lamp in circuit with it remained equally 

 bright. To avoid this possible cause of error the dynamometer, 

 which was constructed by Messrs. Shepherd, Vignoles, and 

 Wheatley, the three of the students of the Central Institution 

 who carried out the investigation, was wound with much finer 

 wire than would usually be employed in the construction of a 

 dynamometer not required to read currents much below one 

 ampere. The dynamometer was for that reason unnecessarily 

 sensitive, and it required a fairly strong spring to control 

 the motion of the suspended coil. This led to an unnecessary 

 waste of energy in the dynamometer, but that was of no con- 

 sequence in this investigation, as our object was to measure 

 the mean square of the current most accurately, and not to 

 satisfy the condition, which is of considerable importance in 

 the design of commercial measuring-instruments, of wasting 

 as little energy as possible in the instruments. 



To the suspended coil of the dynamometer was attached a 

 mirror, and the values of the deflections of the spot of light 

 were determined by direct comparison with the simultaneous 

 readings of an accurately calibrated magnifying spring- 

 ammeter when various direct currents were successively sent 

 through the circuit. The sensibility was such that a deflec- 

 tion of 400 scale-divisions was produced on a scale 68*88 

 inches away, for a current of 2'53 amperes, which corresponds 

 with a deflection of 140 scale-divisions for 1*5 ampere, 



and this was about the usual current passing through the 

 dynamometer in the actual lamp experiments. The value 



