346 KENNELLY AND UPSON— HUMMING TELEPHONE. [Ju'y 20, 



indicated in Fig. 9, for a tube-length commencing at 270 cm. and 

 steadily reduced to 75 cm., with 8.6 volts in the primary circuit and 

 the same instruments as before. Three sets of curves are given, for 

 0.2 ixi. (microfarad), 0.5 /mi., and oc /xi. (condenser short-circuited), 

 respectively. Referring to the pitch lines, it will be seen that there 

 is not much difference between the cases of 00 and 0.5 [xi. The 

 ascending branches of the zig-zags cut the mean frequency line of 

 G"tf at 90, 132.5, 175 and 210 cm. or fairly in conformity with the 

 series 10 -(- 40;;?, as in curve // of Fig. 2. With 0.2 ju,f., however, 

 the intersections with this line are at 100, 145 and 185 cm., or more 

 nearly in conformity with the series 22 + 40111 cm. ; that is, at points 

 displaced about 12 cm. further along the tube. Moreover, the breaks 

 occur at higher frequencies by about 40 '—'. 



As regards primary current strengths, the minima in each series 

 occur at substantially the points where the pitch line intersects ascend- 

 ingly with the G"# line. That is, the minima of co and 0.5 fii. are 

 fairly close together; while those for 0.2 fxi. are displaced about 12 

 cm. further along the tube. Maximum currents occur near breaking 

 points, as usual. 



Effects of Mechanical Changes in Instruments. 

 Observational Series 8. Effects of Modifying the Transmitter. 

 — In order to study the influence of changes in the transmitter upon 

 the humming note, three similar Western Electric transmitters were 

 selected, of standard type and quality, already referred to as T.^, T^ 

 and Tji, in connection with Fig. 7. The receiver, induction-coil, 

 battery and connections were as in previous tests. The comparative 

 results with these three transmitters are shown in Fig. 10, for tube- 

 lengths steadily reduced from 260 to 70 cm. It will be noted that 

 the ascending intersections of the pitch lines all intersect the mean- 

 frequency line of 825 —' in substantial conformity with the series 

 30 -{- 401U, or in accordance with curve I of Fig. 2. The breaking 

 points do not agree. No. 8 always breaking last at a higher pitch. 

 No. 5 next at a medium pitch and No. 11 first at a lower pitch. It 

 may also be noted that in the hum-extinguishing resistance-test of 

 these three transmitters, as given in Table II., and in Fig. 7, their 

 order of succession was the same. 



