Horizontal Intensity of the Earth's Magnetism. 115 



to purely magnetic means, is in principle subject to a circumstance 

 which occasions difficulties — that is, the unavoidable execution of 

 the two necessary observations (of the oscillations and the deflec- 

 tions) at different times. Since the duration of the oscillations 

 and the angle of deflection depend on the horizontal intensity of 

 the terrestrial magnetism, the variations of the latter must be 

 noted down as long as the absolute determination lasts. At the 

 same time the variation-instruments must not be set up in the 

 building in which the absolute measurement is being made. 

 Lastly, the variations in the magnetism of the needle due to 

 changes of temperature must be taken into account. 



In an observatory in which continuous terrestrial magnetic 

 observations are being made, these difficulties are far less per- 

 ceptible than in the case in which the intensity is ascertained for 

 purely physical purposes, as, for instance, for absolute voltaic 

 measurements. 



Just as the determination of the inclination by voltaic means 

 which Weber introduced has facilitated this part of terres- 

 trial magnetic measurements, so also voltaic determination 

 greatly simplifies the determination of the intensity, inasmuch 

 as all observations on which the earth's magnetism has any influ- 

 ence are simultaneously executed. A method shall now be de- 

 scribed which simultaneously effects the absolute measurement 

 of the magnetic intensity and of a voltaic current, and which is 

 characterized by simplicity and convenience. Several years ago 

 Weber tested this method, and, considering the then imperfect 

 state of the instruments, obtained very satisfactory results, which 

 encouraged a further development of the process. Hence last 

 year all the instruments were constructed in a more perfect man- 

 ner, and set up in the magnetic observatory at Gottingen. As a 

 first instance of its application, I will communicate a measure- 

 ment of intensity and of the current, from which it follows that 

 the voltaic method is not only far more convenient than the purely 

 magnetic one, but also seems not at all inferior in accuracy. 



If one and the same voltaic current is passed through a tan- 

 gent-compass, and through a solenoid with bifilar suspension (a 

 bifilar galvanometer, compare Resultate aus den Beob. des magnet. 

 Vereins, 1840, p. 93 et seq.) whose coils when at rest are parallel 

 to the magnetic meridian, the deflection of the two instruments 

 is proportional to the intensity, i ; that of the bifilar galvanometer 

 is also proportional to the horizontal intensity of the earth's 

 magnetism, T ; the deflection of the tangent-compass, however, is 

 inversely as the latter magnitude. If, now, we know the dia- 

 meter of the coils of the tangent-compass, as well as that of the 

 surface contained by the coils of the bifilar galvanometer, and if, 

 moreover, the statical directive force of the suspending-wires of 



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