DUDDELL OSCILLOGRAPH 77 



which has a moving-coil galvanometer. The development of 

 this type in England we owe to Mr. Duddell, and the Duddell 

 oscillograph is the best-known form of the instrument in this country. 

 A powerful electromagnet is used, between whose pole-pieces are 

 placed, in the narrow air-gap,* two parallel strips of phosphor-bronze 

 whose tension may be adjusted. The strips are stretched over bridge- 

 pieces, the plane containing them being along the field. Across the 

 two strips is fixed a small mirror. The current whose wave-form is 

 required passes up one strip and down the other. As a result, the 

 strips move across the field in opposite directions, thereby deflecting 

 the mirror. It is to be noted that here the strips vibrate bodily across 

 the field, like the strings of a violin, while in the particular Blondel 

 instrument which we have described more in detail, there is no bodily 

 vibration of the soft-iron strip, but simply a torsional vibration, the 

 axis of the strip retaining a fixed position in space. 



* Two uir-gaps tire provided in the double oscillograph. 



