3 i4 HERMANN VON HELMHOLTZ 



the partial tones emitted by the second telephone must be 

 considerably altered, while on the other hand the quality of 

 sound remains the same, Hermann found this incompatible with 

 the Theory of Timbre which Helmholtz had propounded in his 

 earlier acoustical observations. 



Helmholtz then showed, in a paper presented to the Academy 

 on July n, 1878, called 'Telephones and Timbre', that if the 

 induction of each circuit, not only on those adjacent to it, but 

 also on itself, were taken into consideration the conclusions 

 made by Hermann exactly confirmed his explanation of the 

 timbre. He proved that the intensities of the induced currents 

 were independent of the frequency, while their phases on the 

 contrary were a little displaced, and at the same time explained, 

 from the mathematical expressions he had formulated, what 

 had already been observed, i.e. that the deep tones of a man's 

 voice generally appeared too weak with ordinary telephones. 

 Helmholtz did not take the reaction of the oscillating iron plates 

 in the receiver into consideration, because its oscillation has 

 a much less amplitude than that of the corresponding plates 

 in the transmitter. If the duration of the induction current is 

 prolonged without external disturbance for more than o-oi 

 second, he finds that with the direct connexion of the two 

 telephones the electric oscillations corresponding with the 

 highest tones and noises do not vary perceptibly in phase, nor 

 in relative strength, from those of the exciting magnetism ; the 

 deeper tones on the contrary may be considerably displaced in 

 their phases, and somewhat diminished in strength. Quality of 

 sound is affected not by means of the electrical motions, but 

 only by the co-vibrating iron plates. 



After the fatigues of the Rectorial year Helmholtz went to 

 Switzerland for rest, in the first place visiting Boll, who was 

 lying very ill at Davos, and wanted to discuss his theory 

 of light and colour sensation some notes on which were 

 published after his death in 1881 with Helmholtz. From Davos, 

 Helmholtz went by Samaden to Pontresina, then by the Italian 

 Lakes to Milan, and so by Nervi to Siena, which he had long 

 desired to see. On September 24 he writes to his wife from 

 Siena : ' To-day and yesterday I have been exploring this old 

 and most singular mountain nest. It lies crowded together 

 upon a hill-top, surrounded by great walls, and traversed by 



