6 PREFACE. 



design an appropriate vibrating telescope. To test the method, a study is 

 made of the vibration of telephonic apparatus. For the first time I have 

 obtained clear-cut interference vibration curves for two identical telephonic 

 systems joined directly in series, while these forms subsided completely 

 when the telephones were joined differentially. Such a system, from another 

 point of view, is an electric dynamometer capable of appreciating an average 

 alternating current well within a microampere. It could, moreover, be 

 synchronized with an external impulse by aid of the Lissajous curves with 

 the same accuracy as two tuning-forks. 



CARL BARUS. 

 Providence, August iQi8. 



