BLONDEL OSCILLOGRAPH 



75 



high as 40,000 per sec., so that the band is easily able to follow the 

 comparatively slow vibrations due to the current whose wave-shape 

 is ivjuired. 



The powerful permanent magnet shown in Fig. 57 corresponds to 

 the controlling magnet of an ordinary needle galvanometer, the soft- 

 iron band corresponds to the needle or needles, and the deflecting 

 coil is represented by two small coils (not shown in Fig. 57), one on 

 fiich side of the gap, the common axis of the coils being normal to 

 the field of the magnet (i.e. normal to the plane of the paper in 

 Fig. 57). Each gap is, of course, fitted with its own coils. If a 

 current is sent through either set of coils, the field of the correspond- 

 ing gap is distorted, the band tends to take up a position with its 

 width along the resultant field, and is thus twisted. The angle of 



-PERMANENT 



FIG. 56. Cam-plate of Synchronous 

 Motor. 



FIG. 57. Magnet of Blondel Oscillo- 

 graph. 



twist is greatest at the middle point of the band, and it is to this 

 point that the tiny mirror is attached. 



Fig. 58 shows the details of construction of the frame which sup- 

 ports the vibrating band. This frame is made of German-silver in 

 order to avoid excessive eddy currents. On each side of the vibrating 

 band is a tapering soft-iron pole-piece, an arrangement which gives a 

 very intense field. The methods of fixing the ends of the strip and 

 of adjusting its tension are clearly shown in the figure. 



The entire lower portion of the frame supporting the band fits 

 into a glass tube filled, or partially filled, with pure castor-oil. This 

 provides the necessary damping, so that the band merely responds to 

 the forced vibrations impressed upon it by the current, but is prac- 

 tically free from superposed natural vibrations (which would cause 



