42 



BELL SYSTEM TECHNICAL JOURNAL 



clearly resolved on a single transit. Sweep speeds which permit resolu- 

 tion of much higher frequencies are employed for visual observation, 

 where the transient component being studied can be found by frequent 

 repetition of the contact operation. As the occurrence of a particular 

 component varies in time of its position in the entire transient, very high 

 sweep speeds are impractical for photography as a prohibitively large 

 proportion of exposures would be blanks. A sweep speed of about 15 

 kilometers per second is about as high as is useful except in some special 

 cases. 



We may commence the discussion by setting up what appears to be 

 a very simple circuit (Fig. 1), a pair of contacts, one of which is con- 



so ^~r 



VOLTS ^=" 



Fig. 1 — Typical relay and contact circuit. 



nected by a length of wire to a relay winding, which is in turn con- 

 nected to one pole of a 50-volt battery. The mate contact and the 

 other pole of the battery are grounded by very short wires. The 

 oscillograph is arranged so that the voltage between the contacts and 

 the current through them can be observed, great care being taken to 

 insure that the added apparatus does not appreciably change the 

 circuit characteristics even at very high frequencies. A low power 

 microscope may be set up to observe the operating area of the contacts. 

 When the contacts close, the first thing that happens is the discharge 

 of the relay structure (a capacity) and of the wire through the contacts. 

 The wire may be thought of as a radio antenna, more or less open- 

 circuited at the load relay winding terminal, and either grounded or 

 opened at the contacts. The wire forms an oscillatory circuit of 

 moderately heavy damping with a surge impedance of about 100 ohms. 

 As it is charged to 50 volts, when the contacts come together an oscilla- 

 tion having a peak current of 0.5 ampere occurs and is over before the 

 steady current through the relay winding has more than started to 

 build up. The frequency of the line oscillation depends on the length 



