Prof. Ayrton and Mr. Mather on Galvanometers. 369 



Section B. 

 Limiting Sensitiveness of Thomson Galvanometers. 



At the Oxford meeting of the British Association (1891), 

 Prof. Schuster read a paper " On the Construction of Delicate 

 Galvanometers/' in which inquiry is made as to the smallest 

 current that can be detected by a non-astatic instrument of 

 1 ohm resistance, and controlled by a field of strength 0*17 

 O.G.S. unit. The calculated minimum current depends on 

 the resolving power of the mirror employed, the smallest 



angle observable being taken as ^-% where A is the wave-length 



La 



of light and d the diameter of the mirror. The result arrived 



at is that 1*5 X 10~ 8 amperes is the smallest current that can 



be detected under the assumed conditions. 



Taking A. as 5*9 X 10 " 5 centimetres we find the minimum 

 angle to be 2*95 X 10~ 5 radians or about 6 1 seconds of arc; 

 this corresponds with O0593 division on a scale at 1000 divi- 

 sions distance. Expressing Prof. Schuster's result in this 

 way we get 3'9 divisions per microampere as the limiting 

 sensibility of a non-astatic galvanometer of 1 ohm resistance 

 when controlled by a field of 0T7 C.G.S. unit, the cavity 

 within the coil being a sphere of 1 centimetre diameter. It 

 is interesting to notice that to obtain a deflexion of 200 divi- 

 sions per micron mpere (a number one might fairly expect to 

 get from a good Thomson instrument having a period of 

 10 seconds and needles about a centimetre long) would 

 necessitate the controlling field of a non-astatic instrument 

 being reduced to less than one-fiftieth of the assumed value, 

 or to 0-003 C.G.S. unit. 



In his calculations Prof. Schuster had no need to take into 

 account the period of vibration of the suspended system, but 

 in the following treatment of the subject period is considered 

 an important factor, and the specific magnetism of the needle 

 is also taken into account. Further, the suspended system is 

 supposed astatic, but this is not essential, for a non-astatic 

 system with an adjustable control would lead to the same 

 results. 



The calculations in this section of the paper were made to 

 ascertain whether some unusually high records of galvano- 

 meter tests which had been sent to us could possibly be correct. 

 The answer was in the affirmative. A similar investigation 

 had previously been made relating to some instruments of the 

 d'Arsonval type, and this gave an answer in the negative. 

 The calculated maximum sensibility came out about ^ of the 

 published value. Subsequent tests made on an actual instru- 



