684 THURAS: RECORDING SEA-WATER SALINITY 



RECORDER 



To modify the Leeds and Northrup recorder for using alter- 

 nating current at low frequency the only change necessary is 

 the substitution of an electro-magnet for the permanent magnet 

 in the galvanometer. Experiments were performed using a 

 laminated magnet of 2.5 sq. cm. cross section and containing 

 2000 turns of wire. All other parts of the galvanometer were 

 unchanged except that the damping coil on the moving system 

 was removed. The moving coil has 147 turns, a resistance of 45 

 ohms and an area of 3.65 sq. cm. The average flux density 

 passing through the moving coil was found for different currents 

 in the stationary coil. These are given in table 1. 



With this galvanometer and the resistances shown in figure 1, 

 critical damping of the galvanometer was obtained with 0.65 

 amperes in the fixed coil (ii) and 0.15 amperes in the bridge circuit 

 (12). . With a change of 0.03 of an ohm (corresponding to 0.01 in 

 salinity), in the resistance marked A, the end of the aluminum 

 pointer of the galvanometer which is 4 cm. long, moved 1.3 mm. 

 This sensitivity is sufficient to permit recording to 0.01 in sa- 

 linity on shipboard. Reducing the current in the fixed coil to 

 0.4 amperes only slightly underdamped the galvanometer and 

 gave almost the same sensitivity. 



The resistances in the two arms of the bridge are a 100 ohm 

 coil and a resistance determined by the ratio of the electrical re- 

 sistance capacities of the two cells. This ratio was determined 

 by repeated measurements of the resistances of the two cells 

 containing the same solution at the same temperature. The 

 ratio of sealed cell to open cell gives 104.18 ohms as the resist- 

 ance of the coil in the second arm of the bridge. The slide wire 

 of the recorder will have a resistance depending on the accuracy 

 required in the measurements of salinity. If the salinity is to 

 be recorded to 0.01 then resistances can be inserted, automatically, 

 in one of the ratio arms so that the slide wire can be used twice 

 throughout the range of salinities measured. 



