446 Messrs. Ayrton and Mather on Galvanometers. 



magnets uniformly magnetized, and if free magnetism exists 

 along the length of the magnet as well as at its ends, our 

 conclusion must, of course, be modified. Indeed, we convinced 

 ourselves by experiments made at the Finsbury Technical 

 College as long back as 1884 that with ordinary magnetic 

 needles it was necessary to place a coil of given diameter 

 somewhat nearer to the needle than was indicated by the 

 formula in our 1890 paper before the deflecting moment 

 changed sign. But since by far the greatest amount of free 

 magnetism on very thin magnets exists near their ends, we 

 do not consider that the theoretical deductions contained in 

 that paper concerning the waste space can have been far 

 wrong. 



We agree with Prof. Holman that uniformity should be 

 observed in describing the sensibility of galvanometers, and 

 we were glad to see that the system for denoting sensibility 

 which was proposed and used by us in our 1890 paper was 

 adopted in the programme of the Naturforscher und Aerzte 

 which met this September in Frankfurt. The t, however, used 

 by Prof. Holman in his proposed list of observed quantities 

 should be the periodic time, and not, as he states, " the time 

 of a single swing.'"' 



Further, in addition to the data respecting resistance, 

 periodic time, and current per millimetre deflexion at given 

 scale distance, mentioned by Prof. Holman as essential in 

 descriptions of sensitive galvanometers, we would point out 

 that it is also important to give the moment of inertia of the 

 suspended system, for unless this be done it is impossible to 

 make the comparisons of various instruments complete. As 

 is well known, the smaller the dimensions and mass of the 

 moving parts, the more excellent will such an instrument 

 appear, when excellence is judged entirely by the deflexion 

 per micro-ampere at constant scale distance and constant 

 period. But for certain purposes it is necessary to compare 

 instruments under conditions of constant controlling moment 

 per unit angle deflexion, because the stability of the zero and 

 trustworthiness of the readings depend on this quantity, as 

 was pointed out on pages 85 and 89 of our 1890 paper ; so 

 that to make this comparison, a knowledge of the moment of 

 inertia of the suspended system is required. 



Other useful particulars of galvanometers are the total 

 volume of the coils, and the decrement, or logarithmic decre- 

 ment, of the oscillations. And since this latter depends on 

 the periodic time and on the condition of the galvanometer 

 circuit, whether closed or open, the damping should be 

 observed with the instrument under the same conditions as 

 when the periodic time was taken. 



