94 On Galvanometers, 



for Thomson galvanometers whose resistances vary between 

 1000 and 10,000 ohms, the time-constant is about 0*0015 

 second. With very high-resistance instruments the time- 

 constant falls to 0*0005 second, in consequence of the in- 

 creasing importance of the insulating covering of the wires. 



IX. Effect of Time and Use on Galvanometer -Needles. 



The following give the values of D, the deflexion in 

 divisions of a scale, placed at a distance from the galvano- 

 meter equal to a length occupied by 2000 scale-divisions, 

 produced by one micro-ampere, the controlling magnet 

 being adjusted to give a periodic time of oscillation equal to 

 10 seconds. 



Double-Coil Astatic Reflecting- Galvanometer. 

 Resistance, 686 ohms. 



D. 



Needles magnetized and tested the same day 1600 



After about two years' use 366 



Needles remagnetized and then used for about one year . 826 



Single- Coil Astatic Reflecting- Galvanometer. 

 Resistance, 26 ohms. 



As obtained from the makers 198 



After about three years' use 170 



Needles remagnetized and tested the same day 243 



Double-Coil Astatic Reflecting- Galvanometer ; Variable 

 Damping. Resistance, 9744 ohms. 



As obtained from the makers 4200 



After about three years' use 3000 



Needles remagnetized and tested the same day 4280 



Rosenthal Non-astatic Reflecting Micro-Galvanometer. 

 Resistance, 44 ohms. (Figs. 8 and 9.) 



As obtained from the makers 25*4 



After four years' use 5-16 



The needles of the first and last of these four galvanometers 

 have lost much more magnetism than have the needles of the 

 second and third galvanometers. As regards the Rosenthal 

 instrument it might have been expected, in view of the powerful 

 nature of its magnetic needle (see fig. 9) and of the fact that 

 the current only acts on small projections of this magnet, that 

 the magnetism would not be much diminished by use. But 

 it has to be remembered that, whereas with the ordinary 

 reflecting-galvanometers the zero-position of the needles is 

 that in which the lines of force due to the coil are at right 



