Johnson provides a little biographical and historical background, but saves plenty of space to describe the crucial experiment and its importance. The book is aimed at the interested layman; it avoids math entirely, and provides a bare minimum to understand the relevant science. In the case of Faraday's demonstration of the effect of magnetism on the polarization of light, I think the shallow treatment makes it hard to understand what's going on, but for the most part the book hits the sweetspot of being interesting and informative.
One of the historical asides that captured my attention was the story of Count Rumford, which I've expanded on with a little google-fu. Born in Massachusetts in 1753, the erstwhile Benjamin Thompson made his fortune the old-fashioned way -- he married a widow 14 years his senior who was the richest landowner in Concord, NH. When the American rebel scum came after him for his Loyalist sympathies, he left town and informed for the British. In England, he was made lieutenant colonel of the King's American Dragoons and sent back to fight in the colonies, but the War was practically over.
Nevertheless, being on the wrong side of the war, he went back to England, where he was knighted. And then moved on to Bavaria where he worked in the service of the Prince-Elector. Here he did most of his scientific work, and was elevated to Reichsgraf von Rumsford (Rumsford being the name under which Concord had been originally incorporated).
His wife having passed away, the Count turned his amorous attentions upon Madame Lavoisier, who had lost her husband to the Reign of Terror -- as Lagrange remarked, "It took them only an instant to cut off his head, but France may not produce another such head in a century."
Anyway, having successfully wooed Madame Lavoisier in a long courtship, the match was ultimately a poor one. As Johnson describes it:
The marriage didn't last. One day, jealous of his solitude, he barred her guests from the house. She retaliated by pouring boiling water ... on his roses. Finally she paid him 300,000 to 400,000 francs to go away.
We salute Count Rumford, heat scientist and pimpin' golddigger.