MEASURING FOKCKS AXD \VKAU IX SWITCHING APPARATUS 495 



Fig. 19 — ^The entire experimental arrangement. 



circles of contact to be easilj^ observed, normal loads on the lens system 

 were 10 and 15 pounds which resulted in contact circles of 0.030 and 0.034 

 inches diameter. 



With normal forces up to 15 pounds and two surfaces in contact, 

 tangential forces up to 7.5 pounds are necessary in order to bring the 

 central lens near the sliding point if the coefficient of friction is near 

 one-quarter. This force was obtained by impressing voltages in the order 

 of 3,000 rms volts on the barium titanate lead titanate hollow cylinder. 

 This cylinder is 4f inches long, and has an outside diameter of 1 inch 

 and an inside diameter of ^ inch. The ceramic was poled in a radial direc- 

 tion and the constants of the material were such that a force of 167 

 pounds could be generated along the length for a clamped driver when 

 a voltage of 3,000 \'olts (4,750 \'olts cm) was used. On the other hand if 

 the driver works against no stress, the expansion in the plated length of 

 4 inches is 0.7 x 10~ inches. 



The actual force applied depends on how much the relative slip be- 

 tween the glass lenses amounts to. To measure this force, a poled lead 

 titanate barium titanate disk is placed between the driver and the 

 metallic bracket which clamps the middle lens as shown by Fig. 18. All 

 the force exerted on the lens has to be exerted through the disk and 

 hence the \-oltage generated by the disk is a measure of the force exerted 

 on the middle lens. This voltage is calibrated by attaching a spring load 

 of known constants and measuring the displacement of the load by means 

 of a microscope. 



Using a 60-cycle driving voltage, a number of sets of disks were run 

 with varying tangential and normal loads and the wear patterns ob- 

 served. Fig. 20 is a photograph (magnified 100 times) for a normal load 



