OF ARTS AND SCIENCES. 237 



the two thinnest diaphragms, which difference is much less than the 

 difference between the maximum current with the two thickest ones. 



Care was taken in all the experiments described in this paper that 

 the magnet of the apparatus should be far below saturation, even with 

 the strongest magnetizing currents which were used. 



Our experiments were next directed to a study of the conditions 

 operative in the magneto receiver, and particularly to ascertain the 

 effect of a varying degree of magnetization in the magnet of the re- 

 ceiver upon the change in its strength when subjected to the action 

 of a slight current of the same order of magnitude as that used in 

 telephony. 



The apparatus employed consisted of a core of soft Norway iron, 

 six inches long and a quarter of an inch in diameter, and provided with 

 a suitable magnetizing coil. A diaphragm was placed opposite one 

 end of the core, as in the usual magneto receiver. This end of the 

 core also carried two coils of fine wire, like the ordinary telephone 

 magnet coil, one having a resistance of about 130 ohms, the other of 

 about 190 ohms. The former of these coils was connected with a 

 storage cell, a fixed resistance, and a key. By means of this appa- 

 ratus a current of suitable magnitude could be caused to traverse the 

 coil, which evidently corresponds to the coil of the ordinary receiver, 

 and which we have called the " line coil." The effect of this cur- 

 rent will be slightly to increase or decrease the magnetization of the 

 bar magnet, as in the receiver itself. The second fine wire coil is a 

 " measuring coil," and is connected in circuit with a ballistic galvanom- 

 eter, from whose deflection when a current was sent through the first 

 fine-wire coil the effect of this current on the magnet of the receiver 

 could be determined. A magnetometer arranged in the usual manner 

 showed the relative strength of the field of the receiver magnet under 

 differing conditions of magnetization. 



The observations consisted in adjusting the magnitude of the cur- 

 rent through the magnetizing coil so as to give a certain deflection of 

 the magnetometer needle. The magnetizing circuit included a suit- 

 able resistance coil, by the use of which any desired current could 

 readily be secured. The key was then pressed, so that a certain 

 current flowed through the line coil. The proper value was given to 

 this current by means of a rheostat. The induction occurring in the 

 measuring coil gave rise to a transient current through this circuit, 

 the value of which was known from the deflection of the needle of the 

 ballistic galvanometer included in it. Of course the key had to be 

 kept down until the observation was completed, and it was also neces- 



