1 70 ELEMENTARY LESSONS ON [CHAP. in. 



mirror. 1 The adjusting magnet enables the operator to 

 bring the reflected spot of light to the zero point at the 

 middle of the scale. The feeblest current passing through 

 the galvanometer will cause the spot of light to shift to 

 right or left. The tiny current generated by dipping 

 into a drop of salt water the tip of a brass pin and a 

 steel needle (connected by wires to the terminals of the 

 galvanometer) will send the spot of light swinging right 

 across the scale. If a powerful lime -light is used, the 

 movement of the needle can be shown to a thousand 

 persons at once. For still more delicate work an astatic 

 pair of needles can be used, each being surrounded by 

 its coil, and having the mirror rigidly attached to one of 

 the needles. 



Strong currents must not be passed through very 

 sensitive galvanometers, for, even if they are not spoiled, 

 the deflections of the needle will be too large to give 

 accurate measurements. In such cases the galvan- 

 ometer is used with a shunt, or coil of wire arranged so 

 that the greater part of the current shall flow through it, 

 and pass the galvanometer by, only a small portion of the 

 current actually traversing the coils of the instrument. 

 The resistance of the shunt must bear a known ratio to 

 the resistance of the instrument, according to the prin- 

 ciple laid down in Art. 353 about branched circuits. 



2O3. Differential Galvanometer. For the pur- 

 pose of comparing two currents a galvanometer is 

 sometimes employed, in which the coil consists of two 

 separate wires wound side by side. If two equal currents 

 are sent in opposite directions through these wires, the 

 needle will not move. If the currents are, however, 

 unequal, then the needle will be moved by the stronger 



1 As concave mirrors are expensive, a plain mirror behind a lens of 

 " suitable focus may be substituted. The thin discs of glass used in 

 mounting objects for the microscope form, when silvered, excellent light 

 mirrors. Where great accuracy is desired a fine wire is placed in the 

 aperture traversed by the beam of light, and the image of this appears 

 when focused on the screen as a dark line crossing the spot of light. 



