certain Dilute Aqueous Solutions. 383 



This method, however, cannot be depended upon except lo 

 give the whole number of vibrations per second. To deter- 

 mine the exact frequency Michelson's method* was employed; 

 but the results were carried to but two decimal places, as the 

 frequency of a fork driven by a mercury contact does not 

 seem to be more constant than this approximation. Now, in 

 order to use this method, the clock must give exactly equal 

 intervals of time ; Prof. Michelson says that the intervals 

 must differ by less than 0'002 second in rating a fork of 128 

 complete vibrations per second. The clock at my disposal 

 had a mercury contact, and I found that working alone I 

 could not adjust this so accurately in the centre of the arc of 

 the pendulum as is required for consistent results. In any 

 case, as Prof. Michelson states, a mercury contact is not very 

 good unless it is quite narrow in the direction of the swing of 

 the pendulum. 



Alter trying many contacts, and working on the subject 

 for some time, I finally devised one that is very simple and 

 has so far given perfect satisfaction. A contact is desired 

 that can be easily adjusted so as to give absolutely equal 

 intervals of time ; so it seemed that the best method would be 

 to use the complete period of the pendulum instead of the 

 half period, as is ordinarily adopted. This would obviate the 

 difficulty of centering the apparatus. 



The construction of the contact is as follows : — A steel 

 watch-spring about 8 cm. long is soldered to a brass collar 

 which is fastened by a screw to one end of a brass plate 4 cm. 

 wide and 10 cm. long. The collar can be rotated about the 

 axis of the screw, and can be clamped tightly at any angle 

 desired. Perpendicular to the flat surface of the spring and 

 about 5 cm. from the collar is soldered a platinum point 

 which, when the spring is slightly depressed, dips into mer- 

 cury in a cup attached to the brass plate but insulated from 

 it. At the same place and perpendicular both to the platinum 

 point and to the length of the spring is soldered the point of 

 a fine steel sewing-needle. This needle-point must be quite 

 fine and well polished. At the further end of the spring is 

 fastened a vane of mica, which dips into oil contained in 

 another cup fastened to the brass plate and is intended to 

 damp the vibrations of the spring. The brass plate to which 

 these parts are attached is fastened to the back of the clock 

 near the top of the pendulum, and in such a position that the 

 needle-point comes near the centre of the arc of the pendulum. 



To the pendulum rod is fastened a light brass collar, on the 

 back of which is soldered a short piece of steel watch-spring 

 * Phil. Mag. [5] xv. pp. 84-87 (1883). 



