DECEMBER 13, 1906| 
WAT ORAS I 
7 
od 
on 
your Aberdeen stone monuments which are well 
worth investigation. 
I hope, also, that Aberdonians will see that the 
necessary work is done. How I wish I could be with 
you to help in it, and renew the pleasures you allowed 
my wife and myself to feel, going about among 
the relics of a long bygone past in your most modern 
motor car. 
Always sincerely yours, 
NorMAN Lockyer. 
APPLICATIONS OF THE MICROPHONE 
PRINCIPLE. 
(AS interesting booklet upon applications of the 
microphone principle has been written by 
Messrs. Jensen and Sieveking, of the physical labo- 
ratories in Hamburg and Karlsruhe.! By the term 
“‘microphone principle ’’? the authors mean all those 
phenomena which are due to the change of ohmic 
resistance between loose contacts. The memoir con- 
tains a very exhaustive collection of what is to be 
found scattered in scientific literature from the time 
of Munck of Rosenschoeld, to the present day. The 
explanation that in loose contacts the nearer approach 
of the particles resulting from the application of pres- | 
sure is the cause of the diminished resistance ob- 
served, is ascribed to du Moncel and Beetz, who gave 
it almost simultaneously, though independently. 
Among the early practical applications of this property | 
of loose contacts was Hughes’s induction balance, 
which is so well known that no lengthy reference 
need be given here. A less known though also in- 
teresting application may, however, be mentioned, 
namely, the demonstration of nodes and antinodes in 
acoustic waves in cylindrical vessels. 
small microphone into the cylinder, Fossati succeeded 
in locating the position of the nodes and antinodes by 
means of a telephone receiver connected with the 
microphone. The sound waves impinging against 
the loose contacts produce a rasping sound in the 
telephone, which vanishes when the microphone 
reaches the position of a node. In a darkened room 
minute sparks may be seen between the microphone 
plates when the microphone is in the position of an 
antinode. 
Another acoustic application of the microphone 
made quite recently by Hebb is the determination of 
the velocity of sound. He uses two parabolic mirrors 
facing each other, and placed on the same axis. The 
one is fixed, and the other can be moved to a greater 
or lesser distance. In the focus of the first or fixed 
mirror is placed a tuning fork and a microphone, in 
the focus of the movable mirror a second microphone. 
The secondary of an induction coil having two primary 
windings is connected to a telephone. The primary 
windings are connected each with a battery and one 
of the microphones. The sound waves of the tuning 
fork act directly on the microphone next to it, and 
the reflected sound waves on the microphone in the 
movable mirror. It is easy to see that the loudness 
of the tone given out by the telephone depends on the 
frequency of the tuning fork, the distance between 
the two mirrors, and the velocity of sound. If both 
microphones receive antinodes at the same time, the 
tone is loudest, and if there is a phase difference of 
half a period between them the tone is weakest. Now 
the phase difference depends on the distance between 
the mirrors, the length of the acoustic wave, and 
the frequency. By first carefully determining the 
latter, and then finding the position of strongest and 
weakest sound, Hebb was able to determine with 
1 Anwendungen des Mikrophenprinzips.". By Chr, Jensen and H. Sieve- 
king. (Hamburg: Graefe and Sille.) 
NO. 1937, VOL. 75] 
By lowering a | 
| tested with coal gas showed great sensitiveness. 
great accuracy the velocity of sound. He found it 
to be 331.29 metres, the probable mean error being 
only 0.04 m. 
The attempts to use the microphone in seismo- 
graphy do not seem to have led to any practical or 
trustworthy result. Rossi, in 1887, used a microphone 
consisting of a silver plate and pointed lever in his 
underground observatory near Rome, and noticed that 
| the telephone gave out sounds which were unmis- 
| takably the effect of seismic movements, and when 
afterwards the apparatus was tranferred to Vesuvius 
and came under Palmieri’s observation, a general 
agreement between the sounds in the telephone and 
the records of the seismograph was observed, but the 
difficulty of separating sounds due to other causes 
seems to have stood in the way of further develop- 
ments. Nevertheless, the authors think that the 
microphone may be made a seismographic instrument 
of great sensitiveness. 
An ingenious application of the microphone for the 
detection of fire-damp has been made in France by 
Hardy. If the sound waves of two pipes of equal 
pitch impinge on microphones connected in series with 
a telephone a clear note is heard, but if one of the 
pipes emits a but slightly different note there will 
be beats heard in the telephone. Now if one pipe is 
on the bank and the other underground, the latter, 
if there be fire-damp, will be blown with air of a 
different density and emit a different note. The tele- 
phone, by sounding beats, will then give warning of 
the presence of fire-damp. The apparatus when 
An 
admixture of but 0.1 per cent. gave three beats in 
| twenty seconds, and an admixture of 1 per cent. gave 
thirty beats in twenty seconds. 
The memoir deals very fully with the use of the 
microphone in telephony, including the production of 
graphic records such as are given by the instruments 
of Nernst, Lieben, Poulsen, and others. Also the 
use of the microphone in wireless telephony is touched 
upon. The most directly useful part of the memoir 
is, however, a very full account of the work done 
by a large number of experimenters in order to ascer- 
tain the best composition of the material forming the 
loose contact of the microphone, its mass, area of 
contact, specific pressure, and other determining 
factors as regards strength and clearness of sound. 
The connection between the microphone and wireless 
telegraphy is not obvious, and the authors deal with 
this part of their subject very briefly. One sentence 
is, however, so interesting that it may, in conclusion 
of this short review, be quoted verbatim. The authors 
say :—‘‘ Already in 1879 Hughes has used the influ- 
ence of spark discharges on microphonic resistances 
‘for wireless telegraphy over a distance of 400 meters.” 
GISBERT Kapp. 
Si BDWARD J. REBD, IK-C.B:, FoR.S. 
HE death of Sir Edward James Reed on 
November 30 brought a long, useful, and highly 
distinguished career to a close. He was in his 
seventy-seventh year—full of activity, with mind 
as vigorous, and interests in life and work as keen, 
as ever. He was an active worker to the end. 
For the greater part of the last half-century he was 
the most prominent naval architect of his time. His 
influence during that long and important period in 
the progress of naval construction was one of the 
most potent forces that shaped its development and 
improvement. That influence was exerted, not only 
by his work and teaching, but also by constant and 
earnest efforts from his earliest days to promote 
the scientific education and training of young men 
