In reading the history now, Ferranti's claim seems exaggerated. At that time Edison and Tesla were fighting it out AC vs. DC. We all know that AC won that fight, but at the time it wasn't clear to everyone. Ferranti bet on AC, as did Westinghouse and everybody that made that bet won big. But it appears that Ferranti got the credit not for being first, but for having the prettiest patent filing.
Ferranti was born in 1864. So he was in his early 20s as all this was ramping up. In his early teens he was a bit of a gear head experimenting with arc lights, and dynamos. Mr. de Ferranti designed a dynamo with William Thomson (aka Lord Kelvin) in old engineering text books they call it the "Ferranti Dynamo." He was supposedly 16 at the time so that would have been around 1880. He started work for Siemens in 1881. Remember the ZBD transformer was developed by 1885 so Ferranti has a small time window to figure this all out and get to court in time to piss off Gaulard and Gibbs.
This Ferranti Dynamo (or it's components) is largely what most history books are claiming is his transformer. Obviously dynamos are not transformers, but they do have some things in common... like those coils. This wasn't his only widget in that era. He was also making alternators and armatures. All of them were metal bodies and wound copper coils. He took out a patent on his modifications to existing transformers in December of 1885: patent #15,141 "Improvements in Electrical Converters." His transformer was made of iron strips laminated with paper on one side, bundled and shellacked. A cast iron frame supported the bundles. Cleverly he also patented his distribution system to set up transformers in groups or parallel mains. Because he got these two ideas on paper, we generally consider it the birth of the modern transformer. John Fleming wrote a comprehensive definition in 1897. I'll paraphrase for brevity:
Fleming described the ZBD transformer as a major improvement on Gaulard and Gibbs. But by 1885 Ferranti was off and patenting specialized transformers for telephony with Addenbrooke. It's important to note that Ferranti had a good lawyer. His lawyer recommended he set up private manufacture of his alternators to keep it out of the hands of Siemens. He later married his lawyers daughter and gained a very useful father-in-law. In 1886 he was appointed chief engineer for Sir Coutts Lindsay & Co. Flush with cash and backers he challenged the Gaulard and Gibbs patents in 1886.1. When the primary circuit has its terminals connected to primary mains it should exhibit always a constant difference of potential between its secondary terminals, independent of the secondary load
2. There should be no waste of energy other than that due to the resistance of the conductors or circuits.
3. There should be perfect and permanent insulation between the primary and secondary coils and between both coils and the iron core.
4. There should be no sensible electrostatic capacity between the primary and secondary coils.
5. When the transformer has its secondary circuit open, no sensible current should flow through the primary circuit from the primary mains.
6. The loss of energy in transformation should be as small as possible at all loads.
The problem was that Gaulard and Gibbs papers and hardware had already made the rounds of the technical magazines. Most patent improvements in that time window start with Gaulard and Gibbs; not Ferranti. Westinghouse had bought half a dozen and shipped them to Pittsburgh. And crucially, William Stanley was in Brooklyn, NY, corresponding with Gaulard and making his own improvements...