CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE 

 ASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



L. — AN APPARATUS FOR RECORDING ALTERNATING 



CURRENT WAVES. 



By Fkank A. Laws. 



Presented May 10, 1900. Received December 15, 1900. 



FiG I 



The apparatus forming the subject of this communication was con- 

 structed at the Rogers Laboratory in 1898, and has proved of sufficient 

 value to merit a short description. In brief, the arrangement gives us a 

 modification of the "contact method," by which the record is rendered 

 continuous and traced photographically. 



The necessary electrical connections are shown 

 in the diagram. K x and K 2 are two rigidly 

 connected contact wheels of ebonite. Into the 

 periphery of each wheel are set four brass blocks. 

 These are accurately placed 90° apart. Upon 

 each wheel a brush and collector ring give per- 

 manent contact with the blocks. Another brush 

 resting on the periphery of the wheel completes 



electrical connection as the blocks pass under it. The brushes are so 

 placed that contact is made and broken at K 2 before K x closes. The 

 contact wheels are driven by a synchronous motor, which gives one rev- 

 olution for four complete alternations of the E.M.F. G is a dead-beat 

 galvanometer, and C is an adjustable condenser. The leads a and b are 

 carried to the points between which the P.D. is to be investigated. By 

 inspection of the diagram it will be seen that once on each wave and at a 

 definite point the condenser C is charged to the potential existing between 

 a and b. As the charge is determined by the breaking of the contact, 

 the blocks may be of sufficient width to eliminate the effect of the jumping 

 of the brushes. Also the resistance at the contact will not be of suffi- 

 cient magnitude to prevent complete charging of the condenser. 



VOL. 



xxxvi. — 21 



