348 BRUSH— DEVELOPMENT OF MAGNETIC SUSCEPTIBILITY 



furnace and solenoid are coincident. The solenoid is excited by 

 current from a 65-volt storage battery, controlled by a rheostat, and 

 the circuit is closed and opened by a switch which breaks simul- 

 taneously at three points in series, so as to avoid the destructive arc 

 which would occur at a single break. An ammeter and reversing 

 switch are included in the line. 



A single turn of asbestos-insulated platinum wire is located in 

 the furnace, and the ends of this loop are connected by a twisted 

 cable with a ballistic mirror-galvanometer of 600 ohms' resistance. 



When the solenoid circuit is closed, a brief electric current is 

 induced in the platinum loop in the furnace and causes a minimum 

 swing of the galvanometer scale easily read with considerable pre- 

 cision. 



When a bundle of ordinary steel or iron bars is placed within the 

 platinum loop the galvanometer deflection is, of course, many times 

 greater, and is fairly proportional to their magnetic susceptibility, 

 after deducting the minimum deflection due to the platinum loop 

 alone, and when the excitation of the solenoid is not too small or 

 too great. In the following experiments with the manganese steel, 

 9 amperes was found to ba suitable exciting current with the solenoid 

 coils in series. Small variations of exciting current were reduced 

 to this value in computing results. Residual magnetism was meas- 

 ured by the usual method of reversals, and allowed for. 



The above described apparatus was originally designed and 

 constructed for a rough study of the magnetic properties of metals 

 and alloys at temperatures up to and above their melting points, and 

 has proved very useful. A high-temperature furnace jvith slotted 

 brass casing is included in the general outfit. 



Preparatory to the following study of magnetic susceptibility of 

 the manganese steel brought about by prolonged heating, ten half 

 inch round bars, 6" long, of Swedish charcoal iron were placed 

 within the platinum loop in the furnace, and the galvanometer de- 

 flection was repeatedly observed when the solenoid was excited by 

 various amounts of current. Nine amperes was found to give con- 

 veniently large deflection, which was closely proportional to the 

 current through a wide range about this value. This condition was 

 also found approximately true when the manganese steel bars, made 



