102 PROCEEDINGS OP THE AMERICAN ACADEMY 



the field actually employed in the magneto-receiver. This was done 

 in the following manner. A magneto hand telephone was so placed 

 that it produced a deflection of 45° in the needle of a delicate mag- 

 netometer. The telephone was then replaced by the magnet and 

 magnetizing coil used in our experimental apparatus, and the current 

 through the coil was varied until a deflection of 45° was produced in 

 the needle. The current producing this effect was found to be 52 

 milliaraperes. A like experiment being performed with a second tele- 

 phone, the value of 62 milliamperes was obtained for the magnetizing 

 current. The latter value was chosen, as it came nearest to a strenirth 

 of current which we had actually used, viz. 77 milliamperes, the low- 

 est magnetizing current that we had employed. 



By the use of the logarithmic method the equation of this curve 

 was found to be xf- = 0.305 rc^*^®, in which y represents the current 

 and X the corresponding amplitude of vibration. Substituting in this 

 equation the value of 2 milliamperes for y, this being the value 

 of a strong telephone current, we obtain the corresponding value 

 of X, which is 2.2 ; that is, the excursion of the diaphragm of a tele- 

 phone receiver with the strength of field corresponding to the curve 

 would be 22 ten-thousandths of a millimeter. But the true excursion 

 is probably somewhat less than this, since the strength of field for 

 which the equation holds is somewhat greater than that emjjloyed in 

 the telephone, and for a magnetization of this value the deflection 

 would be larger with the stronger field. 



It may also be inferred from the preceding results, that so far as 

 sensitiveness is concerned it would be advantageous to employ a 

 stronger magnet in the receiver than is at present used. With the 

 modex'n microphone transmitter this is not necessary, and upon lines 

 where there is much disturbance it would be harmful, as the proper 

 jDrocedure in such cases is to strengthen the transmitter as much as 

 possible, in which case the receiver may be less sensitive. But if a 

 less strong transmitter is used, as, for example, a magneto-telephone, 

 and upon lines free from disturbance, increased sensitiveness in the 

 receiver is very desirable. 



Rogers LABORATORy of Physics, 

 May, 1892. 



