Polytechnic Association. 



949 



under the influence of an increasing number of convolutions, and 

 therefore the influence is more and more increased. 



7. It being evident that the truth lay between these extremes, the 

 expedient of a needle in the form above described was resorted to, and 

 with entire success, for in this the condition sought is accurately fulfilled. 



8. Coil No. 1 is composed of very fine copper wire, wound evenly 

 back and forth over the whole width of the coil, and of a sufficient 

 number of layers to give a resistance of 150 or more ohms. 



No. 2 is of No. 30 wire, wound in the same manner and to twenty- 

 five or thirty ohms risistance. No. 3 is of two layers of No. 23 

 wire, giving one to two ohms resistance. And No. 4 is a strip of 

 sheet copper of the width of the coils, and wound three and a half 

 times around, so that the current passes four times under the needle ; 

 the resistance of this may be considered as null, or not sufficient to be 

 noticed or taken into account. 



9. The outer ends of all the coils are connected with a common 

 screw-cup, while the inner ones are connected each with its cup bearing 

 its proper number. . 



One, two, or even three of the coils may be dispensed with in gal- 

 vanometers for special purposes, according to the function to be 

 performed. 



10. Coil No. 1 is for currents of high intensity, No. 4 for those of 

 great quantity, and Nos. 2 and 3 are for mixed or intermediate cur- 

 rents. 



Galvanometers of different styles are made. 



11. The tangential proportionality of these galvanometers has been 

 tested, on several occasions, in the following manner, with correspond- 

 ing results in all cases. 



Taking galvanometer No. 16, and providing a resistance coil to be 

 put in circuit with coil 3, to make its resistance precisely equal to that 

 of coil 2, and then taking the deflection under different resistances 

 from ten to 500 ohms, and dividing the tangents of the mean deflec- 

 tions, obtained from coil 2, by those from coil 3, we have the following 

 quotients. 



