Form of Harmonic Analyser. 



171 



the glass is marked (on the side next the paper) with a black 

 dot, the "tracing dot." The two remaining windows are 

 provided with reference marks that give with the centre dot 

 a base-line for setting the disk in any desired position. A 

 small conical hole is made in the top of the disk, on a radius 

 perpendicular to its base-line at a distance 10/7T centimetres 

 from the centre. This hole serves to receive the tracing- 

 point of an ordinary Amsler planimeter, which performs the 

 integration. The smaller disks are built up in three layers — 

 a flat bottom plate, the toothed wheel, and a projecting crank, 

 in the top of which is the planimeter hole. This hole is 

 outside the circumference of the toothed wheel, so a crank or 

 some such device is necessary. The crank swings clear 

 above the rack when the wheel is in gear. The windows are 

 arranged as in the first disk. 



§ 5. The ordinary pattern Amsler planimeter with an arm 

 about 16 centim. long does very well, but the tracer must be 

 made vertically adjustable. The alteration is easily made 

 without risk of damage. The planimeter remains of course 

 available for its ordinary purposes, and for the determination 

 of the absolute term. 



The travel of the ruler is limited in two ways : first by the 

 " reach'' of the planimeter, secondly by the risk of running the 

 reading wheel into the rack. Curves to be analysed must 

 consequently be drawn to a moderate scale, but the per- 

 missible magnitude varies with the type of curve. If the 

 type be anything like (1) of fig. 3 (sine type), 10 centim. 



Fiur. 3. 



amplitudes can be taken in comfortably ; but if the type be 

 that of (2) in the same figure, a considerably smaller scale 



