PIERCE. — CRYSTAL RECTIFIERS FOR ELECTRIC CURRENTS. 329 



moved over the sensitive paper 4800 times, without any failure of per- 

 fect superposition, and without any appreciable fogging of the paper. 



The deflecting electromagnets MM had a combined resistance of 436 

 ohms, and were provided with soft iron cores about 6 millimeters in 

 diameter. With these deflecting coils a direct current of 1.5 milliam- 

 peres gave a deflection of 1 cm. on a ground glass put in the place of 

 the sensitive paper at the back of the camera. A calibration for differ- 

 ent values of direct current through the coils showed the deflections of 

 the light spot to be proportional to the current, for the small values of 

 the current employed, and showed no evidence of hysteresis in the 

 iron. 



The Oscillographic Records. — Reproductions (reduced to £) of a 

 characteristic set of the oscillographic records obtained are given in the 

 Plate. Oscillograph No. 1 was taken with the molybdenite rectifier ad- 

 justed to give practically perfect rectification. No. 2 is with the same 

 rectifier slightly out of adjustment (overloaded), so that the rectification 

 is less perfect. No. 3 is with the same rectifier further out of adjust- 

 ment. No. 4 is an oscillographic record with the carborundum rectifier. 

 No. 5 is with the rectifier of brookite. In taking No. 2 the rectifier 

 was submerged in oil, to test the effect of cooling. 



In making these records the following steps were taken : The drum 

 carrying the film was set rotating. The high-potential current was 

 started in the tube. The potential V (Figure 6) and the contact of the 

 rectifier were adjusted so that the deflection of the luminescent spot 

 on the fluorescent screen was wholly or chiefly to one side of the zero 

 position. Exposure of about 2 minutes was then made. This exposure 

 gave the heavy line of the oscillograms. The switch at T was then 

 thrown open, so that the luminescent spot came to its zero position. 

 The exposure in this position was made for a shorter time of about 40 

 seconds. This traced the light straight line along the centre of the 

 picture, and gave the axis of zero current. The switch at T was 

 then thrown to the position to put the resistance R in the circuit in 

 place of the crystal. The resistance R had been previously adjusted 

 so that the amplitude of the deflection with R in the circuit should 

 coincide with the maximum amplitude with the crystal in the circuit. 

 With the resistance R in circuit an exposure of about 1 minute was 

 made, giving the light sinusoidal curve of the picture. 



On each picture the three exposures give, therefore, (1) the form of 

 the rectified cycle as a heavy line, (2) the position of the axis of zero 

 current, as a straight line through the figure, and (3) the form and po- 

 sition of the alternating current cycle when an equivalent resistance R 

 is substituted for the rectifier. The last-named cycle appears in the 



