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Physics. — ''A quick coil galvanometer." By Dr. W. J. H. Moll. 

 (Communicated by Prof. W. H. Julius). 



Galvanometers are applied for the investigation of a great variety 

 of plienomena. The demands which the instrument is to meet, will 

 differ according to the nature of the investigation, and therefore 

 many different kinds have a claim to existence. If we confine 

 ourselves to those cases where the galvanometer has to measure a 

 direct current, and a great sensibility is required, even then the 

 choice of the instrument that will best serve a definite purpose, is 

 often difficult. 



For some methods of investigation, in particular those in which 

 the galvanometer is inserted in a circuit of great resistance, its own 

 resistance will be of little importance. In those cases the string 

 galvanometer is preferable, which is very sensitive, exceedingly quick 

 in its indication, and not easily disturbed. For completeness' sake 

 we must mention a galvanometer (part of the "Kardiograph") con- 

 structed in the course of this year by Siemens and Halske, about 

 which however no particular data have been published as yet. 



In many other methods of investigation, however, the resistance 

 of the circuit is comparatively slight, and the above mentioned 

 galvanometers cease to be sensitive on account of their great resistance. 

 In those cases we must choose between the needle galvanometers 

 and the coil galvanometers. 



Of these two types the needle galvanometers will only be selected 

 for those measurements, for which the utmost sensibility is required. 

 As a matter of fact they are greatly subject to disturbances, and it 

 is extremely diflficult to protect them, especially in cases of great 

 sensibility. 



The coil galvanometer is much simpler in its use and for most 

 purposes sufficiently sensitive. It surpasses the needle galvanometer 

 not only by its being less readily disturbed, but also by the ease 

 with which the damping may be regulated and dead beat conditions 

 attained. These two advantages account for the preference which is 

 generally given to this apparatus. 



The common coil galvanometer is, however, by no means quick ; 

 the time of its indication varies from JO to 30 seconds for the 

 different patterns. This drawback makes itself felt, when the investi- 

 gation requires a long series of readings ; the observation of rapidly 

 varying phenomena is evidently impossible with such a slow instrument, 

 while the string galvanometer, which is exceedingly quick, is generally 

 not sufficiently sensitive on account of its great resistance. 



