April 13, 1871] 
NATURE 
475 


75934 _ 
5. And the work per grain of zinc will be ; 
1427°3 
532 
or $ of the maximum duty. 
III. Lessen the load so that the velocity of the engine 
is increased until the current is reduced to one half its 
original amount, or 2°792 shown on the galvanometer by 
a deflection of 70° 18’. Then we shall have 
1. Heat evolved per hour by the wire 442.64 X G y= 
110'66 units. 
rs) 
Consumption of zinc per hour 2141 X >= 1070°5 
grains. 
3. Heat due to 1070°5 grains, 442°64 X — = 221°32 units. 
I 
2 

4. Therefore the work per hour will be (221°32 — 110°66) 
772 = 85430 ft. lbs. 
5. And the work per grain of zinc will be 85429 = 798 
1070'S 
or $ the maximum duty, 
IV. Let the load be further reduced until the velocity 
reduces the current to 4, or to 18613 shown by a deflec- 
tion of 61° 45’. Then we shall have 
1. Heat evolved per hour by the wire 442°64X ie) = 
49182 units. 
2. Consumption of zinc per hour2141 ce =713'66 grains. 
3. Heat due to 713°66 grains of zinc 442°64X ae 147°55 
units, 
4. Therefore the work per hour will be (147'55 — 49°182) 
772=75940 ft. lbs. 
75940 
5. And the work per grain of zinc will Sree 106"4, 
or % of the maximum duty. 
V. Let the load be still further reduced until, with the 
increased velocity, the current becomes reduced to ;4,, 
or to ‘05584 showing a deflection of 3° 12’. 
Then we 
shall have 
1. Heat evolved per hour by the wire 442°64 X (Ge) 
= 044264 of a unit. 
: : I 
2. Consumption of zinc per hour 2141 X Joo = 20°41 
grains, 
3. Heat due to 21'41 grains of zinc 442°64x som 4°4264 
units. 
4. Therefore the work per hour will be (4°4264 — ‘04426) 
772 = 3383 ft. Ibs. 
5. And the work per grain of zinc will be 333 = 158 
or ;%9, of the maximum duty. 
Now suppose that we still further improve our engine 
by making the stationary magnets twice as powerful. In 
this case all the figures will remain exactly the same as 
before, the only difference being that the engine will only 
require to go at half the velocity in order to reduce the 
current to the same fraction of its first quantity. The 
attraction will be doubled, but the velocity being halved 
no change will take place in the amount of work given out. 
In all cases the maximum amount of work per hour is 
obtained when the engine is going at such a velocity as 
reduces the current to one half of its amount when the 
engine is held stationary ; and in this case the duty per 
grain of zinc is one half of the theoretical maximum. 
The same principles apply equally well when, instead of 
employing the machine as an engine evolving work, we 
do work on it by forcibly reversing the direction of its 
motion. Suppose for instance we urge it with this reverse 


velocity until the quantity of current is quadrupled, or 
becomes 22°386 indicated by a deflection 87° 26’, 
we shall have 
Then 
1. Heat evolved per hour by the wire 442°64 * 42= 
70822 units. 
Consumption of zinc per hour 2141 X4=8564 grains. 
Heat due to 8564 grains of zinc 442°64 X 4 = 1770°56 
units. 
. Therefore the work per hour will be (1770°56— 
7082'2) 772= — 4100432 ft. lbs. 
5. And the work per grain of zinc will be Rae 
= —478'8 or—3 times the maximum working duty. 
The principal reason why there has been greater scope 
for the improvenmt of the steam engine than for the 
electro-magnetic engine arises from the circumstance that 
a 
fo yy 

a applied to the steam engine by 
Thomson, in which a and 4 are the highest and lowest 
temperatures, these values are limited by practical diffi- 
culties. For @ cannot easily be taken above 459°+374° 
=833° from absolute zero, since that temperature gives 
12425 atmospheres of pressure, nor can & be readily 
taken at less than the atmospheric temperature or 449° 
+60°=519°. Also there is much difficulty in preventing 
the escape of heat ; whereas the insulation of electricity 
presents no difficulty, 
I had arrived at the theory of the electro-magnetic 
engine in 1840, in which year I published a paper in the 
4th Vol. of Sturgeon’s Annals, demonstrating that there is 
“no variation in economy, whatever the arrangement of 
the conducting metal, or whatever the size of the battery.” 
The experiments of that paper indicate 36 ft. lbs. as the 
maximum duty for a grain of zinc in a Wollaston battery. 
Multiplying this by 4 to bring it to the intensity of a 
Daniell’s battery, we obtain 144 foot Ibs. Here, as in the 
experiments in the paper on Mechanical Powers of Electro- 
Magnetism, Steam, and Horses, the actual duty is less 
than the theoretic ; which is owing partly to the pulsatory 
nature of the current, and partly also to induced currents 
giving out heat in the substance of the iron cores of the 
electro-magnets ; although these last were obviated as 
far as possible by using annealed tubes with slits down 
their sides. J. P. JOULE 
in the formula 

OBFECT TEACHING AND SCIENCE IN 
AMERICA 
pane following article, reprinted from the Scientific American, 
will give some idea of the spirit in which the teaching of 
science is being pursued in the United States :— 
“The public are beginning to be awakened to the fact that 
technical education is ¢#e education they require, being in ac- 
cordance with the conditions of modern civilisation ; and it is 
admitted that such technical education must be based upon a 
foundation of natural knowledge. The principles of the natural 
sciences must then, for the future, form an essential part of popu- 
lar education ; the only questions are, how far and in what man- 
ner are these sciences to be introduced? Whatever is to be the 
amount taught, educators are agreed that the first steps in 
natural science, or, in other words, in systematising natural know- 
ledge, are to be taken as early as possible. Early impressions 
are the deepest, and every child before its school days is already 
an untrained student of nature. The foundations of technical 
education should, therefore, be laid in the primary school ; but 
whether commenced thus early or not, the method will always 
be the same. The child must be encouraged and guided in its 
natural habits of observing, and it must be led to systematise its 
observations, connecting them together by a chain of reasoning 
into groups of related ideas. This method is simply that known 
as ‘‘ object teaching ;” and you may as well try to fly without 
wings, or to teach geography without maps or globes, as to 
teach natural science without objects and diagrams. There is 
not a teacher, nowadays, but has heard of this object teach- 
ing ; there are hundreds who have tried to utilise it ; there are 
