A chemical balance notices a single milligram. So what keeps such a delicate machine upright, which screw brings the beam back to zero, and where does that tiny rider sit? Here is the Paul Bunge balance, opened up part by part.
1. The base, where everything stands
The Paul Bunge balance has four main parts: the base with its glass box, the column, the beam with its pans, and the rider with its rider carrier.
The base is a rectangle of granite stone or ebonite (hard rubber). A glass box is attached to it, and the main balance stands upright inside that box.
Under the base are three levelling screws: two at the front, right and left, and one at the back in the middle. You level the base by turning the two front screws. When it is level, the spirit-level bubble sits in the middle, or the pointed tip of the pendulum lines up with the fixed pointed tip on the column.
On the right of the base is a handle. It raises and lowers the pans, which is how you make the balance active or close it.
2. The column and the beam
The column is a hollow tube of brass, fixed upright in the middle of the base. Inside it runs a metal rod: its lower end joins the handle rod, and its upper end carries a smooth agate plane. The central knife edge of the beam rests on that plane.
The beam is made of aluminium or brass and is the main part of the chemical balance. It carries three smooth prism-shaped knife edges of agate: one on each side and one at the centre. Two equal pans hang from the side knife edges.
The beam has a screw at each end. Turning them brings the pointer to the zero mark of the main scale at the foot of the column. When the balance is closed, the beam lies on two holders at the ends of the two horizontal rods at the top of the column.
3. The rider and its carrier
The rider is a small loop or twisted wire of Pt or Al, with a mass of 5 mg or 10 mg. It can be moved along the beam from outside the glass box with a metal rod. That rod is the rider carrier.
The rider is taken from the weight box with forceps. With the rider carrier it is placed at the 0 mark of the beam to level the balance. When the pointer comes to 0 on the lower scale, the balance is level and ready to use.
Before the names, see the jobs. The balance leans; tap a part and it does what it is there for.
the balance opened uptap a part
Parts of the Paul Bunge balance
Now see the whole balance on one drawing. Tap a numbered dot on the figure, or a label below it, and the part's name and job appear.
Figure 1.3tap a number or a label
Name the parts
The central knife edge. It rests on the agate plane at the top of the column and carries the whole beam.
The graduated beam. The long bar on top, of aluminium or brass. The rider moves along its marks.
The rider carrier. The metal rod that enters the glass box. It moves the rider from outside.
The rider hook. The hook at the head of the carrier. It holds the rider and sets it on a mark of the beam.
The adjusting screw. At the two ends of the beam. Turning it brings the pointer to the zero mark.
The side knife edge. At each end of the beam, made of agate. A pan hangs from each one.
The pointer. It hangs from the beam down to the lower scale. At the zero mark the balance is level.
The column. A hollow brass tube standing in the middle of the base. The rod inside it carries the agate plane.
The pan. Two equal pans, hung from the side knife edges.
The pan arrest. The name says the job: it holds the pan still.
The base. A rectangular foot of granite or ebonite. The glass box and the main balance sit on it.
The levelling screw. Three under the base. Turn the front two to level the base.
The pan-raising handle. On the right of the base. It makes the balance active or closed.
All thirteen parts seen.
Drawn by hand after the book's Figure 1.3, not to scale.
4. What is each part made of?
After the names, the questions turn to materials. Sort six parts into five boxes.
sort thempart, then material
What is each part made of?
Pick up a part, then drop it in the box for its material. Recall the book's materials.
Right. Next one.
Not yet. Recall what the book says this part is made of.
All in place. Base: granite or ebonite. Column: brass. Beam: aluminium or brass. Knife edges and the plane under the central one: agate. Rider: Pt or Al.
The numbers and short facts are on eight cards. Think of the answer, then turn the card.
quick recallthink, then tap
Quick recall
QuestionHow many main parts?
AnswerFour.
QuestionHow many levelling screws?
AnswerThree.
QuestionHow many knife edges, of what?
AnswerThree, of agate.
QuestionHow many pans?
AnswerTwo equal pans.
QuestionWhat is the mass of the rider?
Answer5 mg or 10 mg.
QuestionWhat is the rod that moves the rider called?
AnswerThe rider carrier.
QuestionHow many front screws are turned to level the base?
AnswerTwo.
QuestionWhere must the pointer be for the balance to be level?
AnswerAt 0 on the lower scale.
5. Check yourself
Three questions in the exam's style. Start with the first.
check yourselfpick one answer
Question 1
Which of the following is NOT applicable to the Paul Bunge balance?
The pointer is on the beam, so it applies.
The rider is part of the Paul Bunge balance.
Right. The book has no part called tare; this is the one that does not apply.
The agate plane is in the book (at the top of the column).
The second asks you to recall the parts.
check yourselfpick one answer
Question 2
At the two ends of the beam there are:
i. Screws ii. Knife edges iii. Pans
Screws and knife edges at the ends; the pans too hang from the end knife edges.
Knife edges are also at the sides. The right choice is i, ii and iii.
Screws are also at the ends. The right choice is i, ii and iii.
Right. Screws, knife edges and pans are all at the two ends.
The last is about the rider.
check yourselfpick one answer
Question 3
What is turned to bring the beam pointer to zero?
It moves the rider. The right answer is: The two screws at the ends of the beam.
These level the base. The right answer is: The two screws at the ends of the beam.
The handle raises and lowers the pans. The right answer is: The two screws at the ends of the beam.
Right. This is the book's method.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionHow many main parts does a Paul Bunge balance have, and which?
AnswerFour: the base with glass box, the column, the beam and pans, the rider and rider carrier
QuestionHow many levelling screws are under the base, and where?
AnswerThree: two at the front (right and left), one at the back in the middle
QuestionHow is it known that the base is level?
AnswerThe spirit-level bubble is in the middle, or the pendulum tip lines up with the fixed pointer tip
QuestionOn what does the central knife edge of the beam rest?
AnswerOn the smooth agate plane at the upper end of the metal rod inside the column
QuestionHow many knife edges does the beam have, where, and of what?
AnswerThree: one on each side and one at the centre; of agate
QuestionWhat is a rider, and what is its mass and metal?
AnswerA movable loop or twisted wire of Pt or Al; mass 5 mg or 10 mg
QuestionWhere is the rider placed to level the balance, and when is it level?
AnswerAt the 0 mark of the beam; when the pointer comes to 0 on the lower scale
Source: Chemistry 1st Paper, Classes 11-12 (NCTB), Chapter 1 “Safe Use of the Laboratory”, section 1.4.1.