THE CALIBRATION OF INSTRUMENTS r,97 



evident by a sudden change of reading when the instrument is 

 tapped. Excessive friction may be due to a cracked jewel, or 

 to other injury, due to dropping the instrument. Again, the 

 freedom of motion of the movable system may be impeded by 

 the buckling of the paper scale, due to dampness, or to projecting 

 fibers of the paper, which cause the pointer to stick. Air 

 dampers, which have very small clearances may also give 

 trouble by getting out of adjustment and lightly dragging on the 

 damping box. In direct-current moving-coil instruments, trouble 

 may be due to dust possibly magnetic, in which case it is hard 

 to dislodge in the air gap. 



Springs. The assumption that the deflections of all sorts of 

 instrument springs are always proportional to the deflecting 

 moment is not tenable. The exact fulfilment of Hooke's law de- 

 pends on the shape of the spring and on the method of mounting. 

 In deflectional instruments the peculiarities of the springs are 

 taken care of in the initial calibration and introduce no trouble 

 unless the spring is subsequently deformed in some manner; 

 but when an equally divided scale and a torsion head are used, 

 as in the Siemen's dynamometer, the instrument must be tested 

 at several points and a calibration curve drawn. 



Zero Shift. This is due to a gradual yielding of the spring when 

 the instrument is kept at a large deflection for a considerable 

 time, an hour or several hours. On breaking the circuit the 

 pointer does not return at once to its original zero position, but 

 will gradually assume it. 



The magnitude of the zero shift depends on the design and 

 material of the spring and on the nearness with which the elastic 

 limit is approached. Springs which are used in high-resistance 

 circuits, such as those of voltmeters and wattmeters, may be 

 made of a material having good elastic properties, such as a 

 bronze, for no limit is set as to spring resistance. Low-resistance 

 springs, for millivoltmeters, have a high percentage of copper and 

 show a larger zero shift than the bronze springs. 



Temperature Coefficients of Springs. With a rise of tempera- 

 ture, the elasticity of the springs decreases about 0.03 or 0.04 

 per cent per degree C. This, if uncompensated, would cause 

 an increase of like amount in the deflection. In many cases 

 electrical and magnetic changes afford a partial compensation. 



