August 16, 1895.] 



SCIENCE. 



195 



alternating current problems the determin- 

 ation of the wave form by the ordinary 

 method of instantaneous contacts is subject 

 to many disadvantages. This method gives 

 at most onlj7 a mean wave curve, correspond- 

 ing to steady conditions. Some continuously 

 recording instrument has long been needed, 

 especially in the case of problems dealing 

 with sudden changes in the conditions. 

 But although numerous forms of apparatus 

 have been suggested and tried, none of 

 the methods proposed appears to be en- 

 tirely satisfactory. The apparatus de- 

 scribed in this article was construed by 

 Messrs. Hotchkiss and Millis for use in the 

 study of sudden changes in an alternating 

 current ; such, for example, as the change 

 brought about by a sudden change in load 

 of a synchronous motor. The apparatus 

 consists essentially of a very light needle, 

 which is merely a mirror mounted on a 

 piece of soft iron, suspended in a rather 

 strong magnetic field. Surrounding the 

 needle is a coil which carries the alternat- 

 ing current to be studied, the axis of the 

 coil being perpendicular to the lines of force 

 of the field. The needle is held in its zero 

 position partly by the magnetic effect, and 

 partly by the torsion of the short fiber, the 

 latter being attached both above and below 

 the needle. Being deflected by the action 

 of the current it indicates current strength 

 by its deflection, and constitutes a true gal- 

 vanometer. A photographic registering de- 

 vice enables a continuous record of the vari- 

 able current to be obtained. 



The essential requirement for accuracy 

 in such an instrument, viz., a short period 

 of vibration, seems to have been filled. The 

 frequency in the case of the five needles 

 used varied from 2850 to 3950 complete 

 vibrations per second. Curves taken from 

 various types of alternating generation are 

 shown, in which no trace of the natural vi- 

 bration of the needle can be seen. Several 

 ' make ' and ' break ' curves are shown also. 



both for alternating and direct currents. 

 Which are of considerable interest. 



The authors call attention to the fact that 

 the apparatus can be used for other purposes 

 besides the study of alternating currents. 

 One interesting example of its application 

 is a curve showing the temperature variation 

 in the interior of a steam-engine cylinder 

 during a single stroke, the temperature 

 measurement depending on the charge in 

 the resistance of a fine iron wire. 

 Experiments with a New Polarizing Photo- Chro- 

 nograph as Applied to the Measurement of the 

 Velocity of Projectiles. By A. C. Crehore 

 and G. O. Squier. 



To avoid the errors due to inertia, which 

 are present in all ordinary types of chi-ono- 

 graph, the present form of instrument is 

 made to depend upon the rotation of the 

 plane of polarization by a current. In the 

 path of a beam of light, which is converged 

 upon a moving photographic plate by a 

 lens, are placed crossed nicols, and between 

 them a tube filled with carbon bisulphide. 

 If current flows in a coil surrounding this 

 tube, light is restored, while if the current 

 is broken, the light reaching the plate is 

 immediately suppressed. The apparatus is 

 especially suited to the measurement of 

 small time intervals. In addition to a de- 

 scription of the instrument, the article con- 

 tains an account of experiments made with 

 it to determine the velocity of projectiles. 

 Experimental Demonstration of a Law of Fluid 

 Pressure. W. J. Humphrey. 

 A description of a simple apparatus for 

 showing that the pressure of a fluid is the 

 same in all directions. 



Boohs Reviewed. — Herts. Die Principien 

 der Mechanik. 8. P. Thompson. Elemen- 

 tary Lessons in Electricity and Magnetism. 

 Yeo. Steam and the Marine Steam Engine. 



THE AMERICAN GEOLOGIST, AUGUST. 



Joseph Granville Norwood, M. D., LL. D. By 

 G. C. Broadhead. 



