366 Prof, Ayrton and Mr. Mather on Galvanometers, 

 Uniformity in recording Tests. 



We would here mention that in some of the galvanometer 

 records we have received, the time of a single swing from one 

 side of the scale to the other has been given as u the period," 

 and this has caused considerable trouble on account of the 

 sensibilities under standard conditions, calculated out on the 

 assumption that the " complete period " was meant, coming- 

 out abnormally high. We therefore think it important that 

 in all records of tests the " complete period," i. e. the time 

 between two transits of the spot across the zero in the same 

 direction, should be given. We may also remark that in all 

 cases the logarithmic decrement (or the decrement) under 

 the conditions in which the period was tested, should be 

 stated, so that the change of period caused by damping may 

 be allowed for. Unless this be done, well-damped instru- 

 ments, which are usually the most convenient, will be unfairly 

 handicapped. In the case of d'Arsonvalor other instruments, 

 in which the periodic time cannot readily be altered by the 

 user, it is important that the actual period of the vibration 

 when tested should be given. 



For a complete record the following particulars are 

 required : — 



(1) Resistance of instrument *. 



(2) Periodic time when tested. 



(3) Divisions deflexion per microampere (or micro- 



amperes per division). 



(4) Scale-distance. 



(5) Length of one scale- division. 



(6) Logarithmic decrement under test conditions. 



(7) Moment of inertia of the suspended system. 



(8) Mass of suspended system. 



(9) Diameter of mirror. 



(10) Length of suspended magnets (perpendicular to 



axis of rotation) . 



(11) Type of instrument. 



(12) Volume of coils. 



(13) Date of test. 



Nomenclature relating to Dead-beat Galvanometers. 



Some ambiguity at present exists in the use of the words 

 " dead-beat " as applied to galvanometers. Sometimes it is used 

 as synonymous with " well-damped " or " aperiodic," whilst 

 at other times the adjective is applied to instruments having 

 quick-moving systems irrespective of whether the decrement 



* If instrument be of the moving-coil type the resistances of coil and 

 suspensions should be stated separately. 



