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17 



basic tiniinji; chafacU'ristics of the slruclui'c, Iml may ^ixc iiisufficiciil 

 information on (lie timing pcrl'ornianco of tlie relay contacts in a paiMicn- 

 lar circuit. For example, contacts may close as desired hut then reoi)en 

 intermittently in a manner to cause either unwanted circuit hehaxior or 

 untlesirahle contact sparking and conseciuiMit erosion. Sometimes the 

 relay armatui'c, in crossing the air-gap, encounters sudden loads as 

 springs are successively picked up in the stroke, causing it to momen- 

 tarily reverse its direction Ix'fore i)ulling home. Also, when the relay 

 armature releases, it may sti'ike the backstop and I'ebound, recrossing a 

 portion of its gaj) and causing undesirable react uating of the contacts. 

 Such momentary opening and closing of the contacts is classed as "con- 

 tact chatter," and special means are needed to detect, measure, and 

 understand its behavior. 



The string oscillograph has been extremely useful for recording and 

 studying such timing effects when the contact chatter was of compara- 

 tiveh' long duration and low freciuency. For relays whose functioning 

 time exceeded 10 milliseconds and where chatter intervals were in the 

 order of 2 or 3 milliseconds each for possibly 6 or 7 successive times, this 

 instrument was most effective. For many of the faster relays, however, 

 where chatter of this type has been completely eliminated, there remain 

 problems of much higher frequency, shorter duration chatter, which are 

 still of great importance to the circuit designer. In such cases the cathode 

 raj'- oscilloscope is used. 



A cathode ray oscilloscope arrangement for the study of chatter is 

 shown in Fig. 11. The horizontal axis of the oscilloscope has a calibrated 

 time base and the trace is displaced vertically to mark closing of the 

 contacts or other events of interest. The horizontal sweep may be trig- 

 gered at the initial closure of the contacts, but a variable time delay 



OSCILLOSCOPE 



Fig. 11 — Cathodp ray oscilloscope circuit for the stu(l,\- of contact chatter 



