MEASURING FORCES AND WEAR IN SWITCHING APPARATUS 473 



graphs with the use of only the amphtiers that are included with such 

 oscillographs. To work down to time intervals in the order of one-tenth 

 second, the hnikage resistance of the load across the polarizetl ceramic — 

 which for small sizes may have a capacitance as low as 20-micro-micro- 

 farads — has to be higher than is usually available in oscillographs. Fig. 

 1 shows a vacuum tube circuit capable of giving a 7oO-mogohm input 

 resistance and when used with a barium titanate ceramic having a 

 capacity of 20 nni, allows measurements of forces for time intervals up 

 to 0.0 b5 seconds with no corrections. This time is usually sufficient to 

 obtain all the force variations in a relay operation. The upper frequency 

 limitation in the measurements of forces is caused by the setting up of 

 natural vibrations in the ceramic block. The lowest frequency vibrations 

 that can be set up in a ceramic bk)ck are the fiexural vibrations. For a 

 l)lock 0.04 inch x 0.04 incli in cross section and 0.02 inch thick, such as 

 have been used in relay force measurements, the lowest fiexural frequen- 

 cies are in the order of l.G x 10 cycles. The next lowest freciuencies are 

 the radial mode vibrations which have frequencies above 4 megacycles 

 for the block considered. Hence the measurements of force should be 

 valid up to times in the order of a microsecond. 



The properties of barium titanate and their stability with time and 

 with large voltages applied in the opposite direction to the poling voltage 

 depend to a large extent on the method of baking the ceramic and on 



lA^AtF 



FOR BALANCING 

 OUT 60 -CYCLE ( 'X, 

 PICK-UP 



10 MEG 



OUTPUT 



:OA/jLF 



'1.8 MEG 



Fig. 1 — High input resistance amplifier tube. 



