Chemistry and Physics. 485 



At its upper end this rod carries an index moving over a gradu- 

 ated scale, calibrated by an electrometer. The resistance of this 

 rheostat varies from 1200 to 5000 ohms. With these resistances 

 in the bridge, the extinction of sound in the telephone was abso- 

 lute. The liquids to be measured were placed on one side of the 

 bridge, using platinum electrodes. With strong or only moder- 

 ately weak solutions, an excellent balance could be obtained ; but 

 when these became more dilute, the error was considerable. With 

 one thousandth solutions of magnesium chloride and potassium 

 chloride, the difference between two consecutive measurements 

 was 2^ per cent by this method; while with the electrometer 

 method, the error was only one-third of one per cent. The au- 

 thors believe, therefore, that for very dilute solutions, the elec- 

 trometer method is preferable. — J. Phys., II, iv, 419, Sept., 1885. 



G. F. B. 



7. A method of precisely measuring the vibratory periods of 

 tuning-forks. — The third volume of the Memoirs of the National 

 Academy of Sciences contains a paper by Professor A. M. Mayer 

 embodying the results of a research recently carried on by him 

 with funds from the Bache endowment. This research had as its 

 object the elaboration of a method for measuring accurately the 

 times of vibration of tuning-forks, and the determination of the 

 laws of their vibrations with reference to the use of the tuning- 

 fork as a chronoscope. The method employed was briefly to 

 make a clock flash, at each second, a spark of induced electricity 

 on a trace made by a style attached to the prong of the vibrating 

 fork. To accomplish this the pendulum of the clock was armed 

 with a triangular piece of platinum foil which each second cut 

 through a globule of mercury contained in a small iron cup. To 

 insure the best results, fresh mercury was taken with each experi- 

 ment and the height of the mercury was adjusted by a screw 

 collar in such a way as to make the globule as nearly as possible 

 rigid and free from vibrations with each touch of the platinum 

 point. The clock through this mercury connection was placed in 

 the circuit of the primary coil of an inductorium, the current of 

 which was given by a single voltaic cell. The tuning-fork, with 

 one of its prongs armed with a light style of thin elastic copper 

 foil, was screwed to a board with a hinge which with a screw- 

 stop suitably placed allowed of its being inclined so that the 

 style was just in contact with a smoked surface of paper wound 

 on a rotating cylinder. The secondary circuit of the induction 

 coil included the fork and cylinder. In the experiment the fork 

 was raised on the hinge, set vibrating by a bow, and then de- 

 pressed again, so that the style should write out its vibration on 

 the smoked surface; at each second, as the platinum-pointed pen- 

 dulum left the mercury, the primary circuit was completed and 

 an induced current caused a spark from the point of the style, 

 which made a single minute circular white spot on the blackened 

 surface. The determination of the vibration-period of the fork is 

 obviously given by counting the number of waves in the trace 



