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Every Class 12 physics student meets Benjamin Franklin on the first pages of the electrostatics chapter, as the person who named positive and negative charge. Most also know the story of the kite. Fewer have read what Franklin actually wrote. His letters are short, clear and online, and they contain the idea behind one of the most useful laws in your syllabus: charge is never created or destroyed. This post uses his own words.
What did Franklin write about plus and minus?
In the 1740s Franklin sent a series of letters on his experiments to Peter Collinson, a Fellow of the Royal Society in London. They were printed in London in 1751 as Experiments and Observations on Electricity, Made at Philadelphia in America.
In one letter he describes three people. A and B stand on wax, which insulates them from the floor; C stands on the floor. A rubs a glass tube and B draws the "fire" from it with his knuckle. Afterwards C, touching neither, feels a spark from each of them. Franklin's explanation is that everyone starts with an equal share of "electrical fire". A has passed some of his to the glass and then to B, so A has less than normal and B has more. Then comes the famous passage: "we say, B, (and bodies like circumstanced) is electrised positively; A, negatively. Or rather, B is electrised plus; A, minus."
A few paragraphs earlier he states the principle behind it: "We had for some time been of opinion, that the electrical fire was not created by friction, but collected". If A and B touch, the spark between them returns things to "the original equality".
How does this connect to conservation of charge?
The NCERT Class 12 Physics chapter on electric charges and fields says the charges "were named as positive and negative by the American scientist Benjamin Franklin". It then states the law in modern form: when we rub two bodies, what one body gains in charge the other loses, and the total charge of an isolated system is always conserved.
Franklin's single "fire" is not how we describe it now. We know that in rubbing, electrons move from one body to the other. But his central claim, that rubbing moves charge rather than making it, is the law of conservation of charge.
Worked example 1: counting electrons
A glass rod rubbed with silk gains a charge of +8.0 × 10⁻⁹ C. (These are sample numbers. NCERT's convention is that glass becomes positive and silk negative.)
Charge on the silk. By conservation, the silk has −8.0 × 10⁻⁹ C.
Electrons moved. n = q/e = 8.0 × 10⁻⁹ / 1.6 × 10⁻¹⁹ = 5.0 × 10¹⁰ electrons, moved from the glass to the silk.
Check. Total charge = +8.0 × 10⁻⁹ − 8.0 × 10⁻⁹ = 0, as before rubbing.
Worked example 2: sharing charge between spheres
Two identical metal spheres carry +6 nC and −2 nC. They are touched together and separated.
Total before. +6 + (−2) = +4 nC.
After. Identical spheres share equally, so each has +2 nC.
Check. +2 + 2 = +4 nC. Charge is conserved.
This is Franklin's A and B touching, in numbers.
What did Franklin's kite letter say?
Franklin described the kite in a letter dated Philadelphia, 1 October 1752, read at the Royal Society on 21 December 1752 and printed in the Philosophical Transactions. He wrote that news had arrived of "the success of the Philadelphia experiment for drawing the electric fire from clouds by means of pointed rods of iron erected on high buildings", and that the same experiment had succeeded in Philadelphia "in a different and more easy manner".
He described a kite made of a silk handkerchief on a cedar cross, with a sharp-pointed wire at the top. A key was tied where the twine met a silk ribbon held in the hand, and the person holding it had to stand under cover so the ribbon stayed dry. When rain wet the twine, electric fire would "stream out plentifully from the key on the approach of your knuckle", and a Leyden phial could be charged from the key. That, he wrote, showed "the sameness of the electric matter with that of lightning".
Never try this. Flying a kite near a thunderstorm can kill you. The point of the letter for us is the reasoning: if lightning charges a phial exactly as a rubbed glass tube does, it is the same electricity.
Common mistakes in conservation of charge problems
Creating charge from rubbing. If one object gains +q, the other must have −q.
Sharing equally between different spheres. Equal sharing only applies to identical conductors.
Losing the sign. Add charges with their signs before sharing.
Conservation of charge is in the electrostatics unit of the JEE (Main) 2026 syllabus. For the traps examiners set, see our guide to common electrostatics mistakes.
If you keep losing signs in charge problems, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
What can a student learn from how Franklin worked?
Franklin explained a confusing set of sparks with one simple, testable idea, an equal share of something that can be moved but not made, and then invented words to keep track of it. When a topic feels messy, look for the one quantity that is conserved, and track where it goes.
Practice routine:
Three identical spheres carry +9, −3 and 0 nC. Touch the first two, then the second and third. Find every final charge.
Find how many electrons make up −1.0 μC.
Read the plus-and-minus passage in Franklin's letter and redraw it as a diagram with charges marked.
Next step: solve problem 1 and check that the total charge is the same at every step.
