SCIENCE AND PRACTICE. 



247 



galvanometer, the angle of deflection is read off on a card, 

 inside the compass-box, with the shorter needle ; when used 

 as a sine instrument the other needle is used, and the circular 

 plate, with the coils, turned in the same direction as the 

 needle is deflected, until they coincide. 



The thicker coil, whose resistance is about one-tenth of an 

 unit, consists of sixteen convolutions of copper wire 1*34 

 millimetres diameter ; the other coil has above a hundred 

 and fifty units resistance, and consists of over a thousand 

 convolutions of insulated copper wire of 0'25 millimetres 

 diameter. 



Should the deflection due to a current be too great to be 

 read off, an arrangement is adopted by which a known 

 fraction only of the current goes through the galvanometer, 

 the other going through a shunt. This is done by inserting 

 the shunting circuit parallel to the galvanometer coil. 



21. Weber's Mirror Galvanometer. In most of his experi- 

 mental researches in galvanism, Professor Weber has em- 

 ployed a galvanometer, the magnetic needle of which is a 

 circular steel mirror reflecting the divisions of an ill mm* - 

 nated scale placed at some distance from it into a telescope 



Fig. 127. 



through which the observer reads off the deflections of the 

 mirror. Its principle is precisely that of the receiving in- 

 strument used in Gauss and "Weber's telegraph already 

 explained. 



