The Babcock Milk-tester
It is not sufficient for a farmer or a dairyman to know how much milk
each of his cows yields. He should also know how rich the milk is in
butter-fat. Wide-awake makers of butter and cheese now buy milk, not by
the pound or by the gallon, but by the amount of butter-fat contained in
each pound or gallon of milk. A gallon of milk containing four and a
half per cent of fat will consequently be worth more than a gallon
taining only three per cent of fat. So it may happen that a cow
giving only two gallons of milk may pay a butter-maker more than a cow
giving three gallons of milk. Of course it is easy to weigh or measure
the quantity of milk given by a cow, and most milkers keep this record;
but until recent years there was no way to find out the amount of fat in
a cow's milk except by a slow and costly chemical test. Dairymen could
only guess at the richness of milk.
In 1890 Dr. S. M. Babcock of the Wisconsin Experiment Station invented a
wonderful little machine that quickly and cheaply measures the fat in
milk. Few machines are more useful. So desirous was Dr. Babcock of
helping the farmers that he would not add to the cost of his machine by
taking out a patent on his invention. His only reward has been the fame
won by the invention of the machine, which bears his name. This most
useful tester is now made in various sizes so that every handler of milk
may buy one suited to his needs and do his own testing at very little
The operation of the machine is very simple. Suppose that the members of
the class studying this book have been asked to take a Babcock machine
and test the milk of a small herd of cows. They can readily do so by
following these directions:
While the milk is still warm from the first cow to be tested, mix it
thoroughly by pouring it at least four times from one vessel to another.
A few ounces of this mixed milk is then taken for a sample, and
carefully marked with the name of the cow. A number is also put on the
sample, and both the cow's name and the number entered in a notebook. A
small glass instrument, called a pipette, comes with each machine. Put
one end of the pipette into the milk sample and the other end into the
mouth. Suck milk into the pipette until the milk comes up to the mark on
the side of the pipette. As soon as the mark is reached, withdraw the
pipette from the mouth and quickly press the forefinger on the mouth
end. The pressure of the finger will keep the milk from running out.
Then put the lower end of the pipette into one of the small long-necked
bottles of the machine, and, lifting the finger, allow the milk to flow
gently into the bottle. Expel all the milk by blowing through the
The next step is to add a strong, biting acid known as sulphuric acid to
the test-bottle into which you have just put the milk. A glass marked to
show just how much acid to use also comes with the machine. Fill this
glass measure to the mark. Then pour the acid carefully into the
test-bottle. Be sure not to drop any of the acid on your hands or your
clothes. As the acid is heavier than the milk, it will sink to the
bottom of the bottle. With a gentle whirling motion, shake the bottle
until the two fluids are thoroughly mixed. The mixture will turn a dark
brown and become very warm.
Now fill the other bottles in the same way with samples drawn from
different cows. Treat all the samples precisely as you did the first. Do
not forget to put on each sample the name of the cow giving the milk and
on each test-bottle a number corresponding to the name of the cow.
You are now ready to put the test-bottles in the sockets of the machine.
Arrange the bottles in the sockets so that the whirling frame of the
machine will be balanced. Fit the cover on the machine and turn the
handle slowly. Gradually gain in speed until the machine is whirled
rapidly. Continue the turning for about seven minutes at the speed
stated in the book of directions.
After this first turning is finished, pour enough hot water into each
test-bottle to cause the fat to rise to the neck of the bottle. Re-cover
the machine and turn for one minute. Again add hot water to each bottle
until all the fat rises into the neck of the bottle and again turn one
There remains now only the reading of the record. On the neck of each
bottle there are marks to measure the amount of fat. If the fat inside
the tube reaches only from the lowest mark to the second mark, then
there is only one per cent of fat in this cow's milk. This means that
the owner of the cow gets only one pound of butter-fat from each
hundred pounds of her milk. Such a cow would not be at all profitable to
a butter-seller. If the fat in another test-bottle reaches from the
lowest mark to the fourth mark, then you put in your record-book that
this cow's milk contains four per cent of butter-fat. This record shows
that the second cow's milk yields four pounds of fat to every hundred
pounds of milk. This cow is three times more valuable to a butter-maker
than the first cow. In the same way add one more per cent for each
higher mark reached by the fat. Four and one-half per cent is a good
record for a cow to make. Some cows yield as high as five or six per
cent but they do not generally keep up this record all the year.
The tester, acid, acid measure, test-bottle, and thermometer at bottom;
filling the pipette on right; adding the acid and measuring the fat at
The Babcock tester shows only the amount of pure butter-fat in the milk.
It does not tell the exact amount of finished butter which is made from
100 pounds of milk. This is because butter contains a few other things
in addition to pure butter-fat. Finished and salted butter weighs on an
average about one sixth more than the fat shown by the tester. Hence to
get the exact amount of butter in every 100 pounds of milk, you will
have to add one sixth to the record shown by the tester. Suppose, for
example, you took one sample from 600 pounds of milk and that your test
showed 4 per cent of fat in every 100 pounds of milk. Then, as you had
600 pounds of milk, you would have 24 pounds of butter-fat. This fat,
after it has been salted and after it has absorbed moisture as butter
does, will gain one sixth in weight. As one sixth of 24 is 4, this new 4
pounds must be added to the weight of the butter-fat. Hence the 600
pounds of milk would produce about 28 pounds of butter.
1. Find the number of pounds of butter in 1200 pounds of milk that
tests 3 per cent of butter-fat.
2. A cow yields 4800 pounds of milk in a year. Her milk tests 4 per
cent of butter-fat. Find the total amount of butter-fat she yields.
Find also the total amount of butter.
3. The milk of two cows was tested: one yielded in a year 6000
pounds of milk that tested 3 per cent of fat; the other yielded
5000 pounds that tested 4 per cent. Which cow yielded the more
butter-fat? What was the money value of the butter produced by each
if butter-fat is worth twenty-five cents a pound?